Add Accessibility Object Model to chrome://flags

This is important for testing on Chrome OS and Android where it's not
always possible to run Chrome with a command-line switch.

Bug: 746523
Change-Id: I8326cdaa44b6013dd0b6a9d1c1f7ca1022cb208e
Reviewed-on: https://chromium-review.googlesource.com/1072505
Reviewed-by: John Abd-El-Malek <[email protected]>
Commit-Queue: Dominic Mazzoni <[email protected]>
Cr-Commit-Position: refs/heads/master@{#561778}
10 files changed