Add BrowserTaskQueues::EnableAllQueues

After this patch all queues in BrowserTaskQueues will be disabled on
creation and users must explicitly enable them via a call to either
BrowserTaskQueues::EnableAllQueues or
BrowserTaskQueues::EnableAllExceptBestEffortQueues

This will help us ensure the invariant that no tasks will run before
BrowserMainLoop::CreateThreads still holds after we add an IO
scheduler.

Bug: 863341
Change-Id: I07d34ec84e166b12b9ab50448acde7f943f9cbbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611979
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#661772}
12 files changed