[chromium] Add enter/exitRunLoop to WebThread API

[chromium] Add enter/exitRunLoop to WebThread API
https://bugs.webkit.org/show_bug.cgi?id=76012

Reviewed by Darin Fisher.

This adds those two APIs to WebKit::WebThread and converts CCLayerTreeHostTest over to use these APIs instead
of webkit_support. The immediate motivation is that we can't use webkit_support in webkit_unit_tests in the
component build.

* WebKit.gyp:
* public/platform/WebThread.h:
* tests/CCLayerTreeHostTest.cpp:
(WTF::CCLayerTreeHostTest::onEndTest):
(WTF::CCLayerTreeHostTest::TimeoutTask::run):
(WTF::CCLayerTreeHostTest::BeginTask::BeginTask):
(WTF::CCLayerTreeHostTest::BeginTask::~BeginTask):
(WTF::CCLayerTreeHostTest::BeginTask::run):
(WTF::CCLayerTreeHostTest::runTest):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106044 268f45cc-cd09-0410-ab3c-d52691b4dbfc