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_unittest.cc b/webrtc/rtc_base/buffer_unittest.cc
index bd4306e..7bf1b5b 100644
--- a/webrtc/rtc_base/buffer_unittest.cc
+++ b/webrtc/rtc_base/buffer_unittest.cc
@@ -10,7 +10,7 @@
 
 #include "webrtc/rtc_base/buffer.h"
 
-#include "webrtc/rtc_base/array_view.h"
+#include "webrtc/api/array_view.h"
 #include "webrtc/rtc_base/gunit.h"
 
 #include <type_traits>