Clearly deprecate blink::WaitableEvent.

The "TODO: Deprecate" was ambiguous. I think the intent is that new code
shouldn't use it and the TODO is to migrate existing code.

Bug: 796799
Change-Id: I73b135b937d65649ff2f0c5fc99723353065e9bb
Reviewed-on: https://chromium-review.googlesource.com/1184136Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585203}

third_party/blink/renderer/platform/waitable_event.h

@@ -41,11 +41,11 @@ class WaitableEvent;
 
 namespace blink {
 
-// TODO(crbug.com/796799): Deprecate blink::WaitableEvent and use
-// base::WaitableEvent instead.
+// DEPRECATED, use base::WaitableEvent instead of blink::WaitableEvent.
 //
 // Provides a thread synchronization that can be used to allow one thread to
 // wait until another thread to finish some work.
+// TODO(crbug.com/796799): Remove this class in favor of base::WaitableEvent.
 class PLATFORM_EXPORT WaitableEvent {
  public:
   // If ResetPolicy::Manual is specified on creation, to set the event state