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/BUILD.gn b/webrtc/rtc_base/BUILD.gn
index 03e8ea6..1914b27 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -835,7 +835,6 @@
       #visibility = [ "..:rtc_unittests" ]
     }
     sources = [
-      "array_view_unittest.cc",
       "atomicops_unittest.cc",
       "base64_unittest.cc",
       "basictypes_unittest.cc",
@@ -883,6 +882,7 @@
       ":rtc_base_tests_main",
       ":rtc_base_tests_utils",
       ":rtc_task_queue",
+      "../api:array_view",
       "../system_wrappers:system_wrappers",
       "../test:test_support",
     ]