Remove LifecycleNotifier<>::m_context.

It serves no purpose any longer to keep this self reference around,
hence retire m_context.

Also, move isContextThread() up to ExecutionContext; a more natural place
to position the predicate.

R=haraken
BUG=462949

Review URL: https://codereview.chromium.org/1008113002

git-svn-id: svn://svn.chromium.org/blink/trunk@191883 bbb929c8-8fbe-4397-9dbb-9b2b20218538
17 files changed