GitLab

Adding labels for images inside the webview. Also fix a visibility issue
for ChromeVox by setting the webview and the image unselectable.

Bug: b/142957550
Test: Manual Test
Change-Id: I98d1826c5f302df0a696dc9c7a75e10799302720
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887848Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710539}