Fix header guard in thread_annotations.h.

It was using a non-WebRTC-named header guard, which could conflict with
other similarly named/intended headers.

BUG=None
NO_DEPENDENCY_CHECKS=true

Review-Url: https://codereview.webrtc.org/2548113002
Cr-Original-Commit-Position: refs/heads/master@{#15554}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 580df537ce829818448f0563c8aded6fc63b886b
diff --git a/base/thread_annotations.h b/base/thread_annotations.h
index 8425030..8d5abbd 100644
--- a/base/thread_annotations.h
+++ b/base/thread_annotations.h
@@ -16,8 +16,8 @@
 // of their multi-threaded code. The annotations can also help program
 // analysis tools to identify potential thread safety issues.
 
-#ifndef BASE_THREAD_ANNOTATIONS_H_
-#define BASE_THREAD_ANNOTATIONS_H_
+#ifndef WEBRTC_BASE_THREAD_ANNOTATIONS_H_
+#define WEBRTC_BASE_THREAD_ANNOTATIONS_H_
 
 #if defined(__clang__) && (!defined(SWIG))
 #define THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))
@@ -97,4 +97,4 @@
 #define NO_THREAD_SAFETY_ANALYSIS \
   THREAD_ANNOTATION_ATTRIBUTE__(no_thread_safety_analysis)
 
-#endif  // BASE_THREAD_ANNOTATIONS_H_
+#endif  // WEBRTC_BASE_THREAD_ANNOTATIONS_H_