Pending tasks in a message loop should be deleted before shutting down Blink

Currently Blink is shut down before all the pending tasks in the message loop are deleted. This is problematic in Oilpan because a destructor of the pending tasks can touch Oilpan objects. Because Oilpan is already detached from the renderer thread at that point, touching Oilpan objects in the destructor leads to a crash. (See the bug report for a concrete scenario.)

To prevent Blink objects from getting accessed after Blink is shut down, this CL deletes all pending tasks in a message loop before shutting down Blink.

TEST=None. I cannot reproduce the crash.

Cr-Commit-Position: refs/heads/master@{#296697}

Cr-Commit-Position: refs/heads/master@{#297338}

Review URL:

Cr-Commit-Position: refs/heads/master@{#303557}
5 files changed