GitLab

Three related changes:
* Only run idle tasks in the queue, not newly added ones.
* Move g_request_pending logic to SharedRendererState. This
  allows pending tasks to be cancelled correctly.
* Wait for kModeProcess to happen first before trying to
  request another to avoid a queue of pending tasks.

BUG=401712

Review URL: https://codereview.chromium.org/444173003

Cr-Commit-Position: refs/heads/master@{#288285}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288285 0039d316-1c4b-4281-b951-d872f2087c98