Make WebTraceLocation wrap a TraceLocation

Make WebTraceLocation wrap a TraceLocation to make their relationship
clearer.

BUG=391005

Review URL: https://codereview.chromium.org/675213002

git-svn-id: svn://svn.chromium.org/blink/trunk@184465 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Clean up forward declarations in Source/platform

These forward references are not needed anymore. After
this patch, Source/platform has no unneeded forward references
any more.

Review URL: https://codereview.chromium.org/644913009

git-svn-id: svn://svn.chromium.org/blink/trunk@184466 bbb929c8-8fbe-4397-9dbb-9b2b20218538