GitLab

Add shortcut keys AltGr+Left/Right for nagivating back and forth among
pages. This allows users of some European keyboards to navigate with one
hand key operations.

BUG=613201

Change-Id: I5eceeaacf0df8029f2b1d6f7dd6a2d85e1100682
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993845Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732678}