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It is possible for the TaskRunner to which the tasks were posted
to be destroyed before the RenderWidgetResizeHelper is destroyed.
Make sure that RenderWidgetResizeHelper is robust to this by having
tasks remove themselves from RenderWidgetResizeHelper's queue
when they are destroyed.

BUG=394775
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/399213003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284224 0039d316-1c4b-4281-b951-d872f2087c98