Oilpan: remove invalid asserts from ListHashSet tracing.

When conservatively tracing ListHashSets we can end up tracing the
ListHashSet itself where the ListHashSet backing has been marked
and is on the marking stack for later tracing. Therefore, the
asserts that the backing has already been fully traced if the
backing is marked when we trace the ListHashSet is invalid.

R=erik.corry@gmail.com, oilpan-reviews@chromium.org
BUG=375669

Review URL: https://codereview.chromium.org/336693007

git-svn-id: svn://svn.chromium.org/blink/trunk@176110 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: Make MediaControllers ExecutionContext a weak pointer.

Cause of a flaky ASAN use-after-poison in media/video-frame-accurate-seek.html

R=ager@chromium.org, haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/333813004

git-svn-id: svn://svn.chromium.org/blink/trunk@176111 bbb929c8-8fbe-4397-9dbb-9b2b20218538