Have AutofillManager obtain the blocking pool that it uses from AutofillDriver

Instead of using content::BrowserThread's global SequencedWorkerPool to execute
blocking tasks, AutofillManager now obtains a blocking pool from AutofillDriver.

BUG=303014

Review URL: https://codereview.chromium.org/26261002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228155 0039d316-1c4b-4281-b951-d872f2087c98
Ignore WasResized if the RWHImpl's delegate has already been detached.

BUG=304847

Review URL: https://codereview.chromium.org/26321004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228156 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228156