Rename RTC_EXPORT to RTC_OBJC_EXPORT.

A new version of RTC_EXPORT will be introduced by [1] and it will be
used by WebRTC native code.

This CL renames the current RTC_EXPORT to RTC_OBJC_EXPORT in order
to avoid to mix them. It has been decided to avoid to unify them because
RTC_OBJC_EXPORT always marks symbols with default visibility, while
RTC_EXPORT will do it only when COMPONENT_BUILD is defined.

[1] - https://webrtc-review.googlesource.com/c/src/+/97960 is

Bug: webrtc:9419
Change-Id: I56a3fc6601c72d3ad6a58f9961a00e3761dfb5da
Reviewed-on: https://webrtc-review.googlesource.com/100521
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24754}
diff --git a/sdk/objc/components/renderer/metal/RTCMTLNSVideoView.h b/sdk/objc/components/renderer/metal/RTCMTLNSVideoView.h
index 891b377..ffed4b8 100644
--- a/sdk/objc/components/renderer/metal/RTCMTLNSVideoView.h
+++ b/sdk/objc/components/renderer/metal/RTCMTLNSVideoView.h
@@ -14,7 +14,7 @@
 
 NS_AVAILABLE_MAC(10.11)
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCMTLNSVideoView : NSView <RTCVideoRenderer>
 
 @property(nonatomic, weak) id<RTCVideoViewDelegate> delegate;
diff --git a/sdk/objc/components/renderer/metal/RTCMTLVideoView.h b/sdk/objc/components/renderer/metal/RTCMTLVideoView.h
index ff64ba3..36cb144 100644
--- a/sdk/objc/components/renderer/metal/RTCMTLVideoView.h
+++ b/sdk/objc/components/renderer/metal/RTCMTLVideoView.h
@@ -26,7 +26,7 @@
  */
 NS_CLASS_AVAILABLE_IOS(9)
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCMTLVideoView : UIView<RTCVideoRenderer>
 
 @property(nonatomic, weak) id<RTCVideoViewDelegate> delegate;
diff --git a/sdk/objc/components/renderer/opengl/RTCEAGLVideoView.h b/sdk/objc/components/renderer/opengl/RTCEAGLVideoView.h
index 50bbf2e..c35fc2d 100644
--- a/sdk/objc/components/renderer/opengl/RTCEAGLVideoView.h
+++ b/sdk/objc/components/renderer/opengl/RTCEAGLVideoView.h
@@ -19,7 +19,7 @@
 
 @class RTCEAGLVideoView;
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCEAGLVideoViewDelegate <RTCVideoViewDelegate>
 @end
 
@@ -27,7 +27,7 @@
  * RTCEAGLVideoView is an RTCVideoRenderer which renders video frames in its
  * bounds using OpenGLES 2.0 or OpenGLES 3.0.
  */
-RTC_EXPORT
+RTC_OBJC_EXPORT
 NS_EXTENSION_UNAVAILABLE_IOS("Rendering not available in app extensions.")
 @interface RTCEAGLVideoView : UIView <RTCVideoRenderer>
 
diff --git a/sdk/objc/components/renderer/opengl/RTCNSGLVideoView.h b/sdk/objc/components/renderer/opengl/RTCNSGLVideoView.h
index ad47705..2540f38 100644
--- a/sdk/objc/components/renderer/opengl/RTCNSGLVideoView.h
+++ b/sdk/objc/components/renderer/opengl/RTCNSGLVideoView.h
@@ -21,11 +21,11 @@
 
 @class RTCNSGLVideoView;
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCNSGLVideoViewDelegate <RTCVideoViewDelegate>
 @end
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCNSGLVideoView : NSOpenGLView <RTCVideoRenderer>
 
 @property(nonatomic, weak) id<RTCVideoViewDelegate> delegate;
diff --git a/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h b/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h
index a570129..6876cc3 100644
--- a/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h
+++ b/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h
@@ -18,7 +18,7 @@
  * RTCVideoViewShading provides a way for apps to customize the OpenGL(ES) shaders used in
  * rendering for the RTCEAGLVideoView/RTCNSGLVideoView.
  */
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCVideoViewShading <NSObject>
 
 /** Callback for I420 frames. Each plane is given as a texture. */