Only consider first submit button in a form as default

Test: with ChromeVox and chrome://settings
Bug: 910792
Change-Id: I8531ee913be0c62e177f207d2f1ef28d4b28d4f3
Reviewed-on: https://chromium-review.googlesource.com/c/1374490
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616454}
18 files changed