Disable posting to the Blink scheduler from callOnMainThreadFunction

Disable posting to the blink scheduler from callOnMainThreadFunction due
to a bug dealing with PartitionAlloc shutdown.

BUG=422841
TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/669613004

git-svn-id: svn://svn.chromium.org/blink/trunk@184022 bbb929c8-8fbe-4397-9dbb-9b2b20218538