Move array_view.h to webrtc/api/

We use ArrayView in our public API, so its header should be in
webrtc/api/.

BUG=none

Review-Url: https://codereview.webrtc.org/3007763002
Cr-Commit-Position: refs/heads/master@{#19658}
diff --git a/webrtc/rtc_base/buffer.h b/webrtc/rtc_base/buffer.h
index 25d55e0..b8fe98c 100644
--- a/webrtc/rtc_base/buffer.h
+++ b/webrtc/rtc_base/buffer.h
@@ -17,7 +17,7 @@
 #include <type_traits>
 #include <utility>
 
-#include "webrtc/rtc_base/array_view.h"
+#include "webrtc/api/array_view.h"
 #include "webrtc/rtc_base/checks.h"
 #include "webrtc/rtc_base/type_traits.h"