Add children() and FindChild() methods to View.

Make use of these in a few places to reduce use of index-based APIs or improve
efficiency.

Bug: 940135, 942969
Change-Id: I879bec72ed25bfb2b2a1ee508fb798f76124f72f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526628
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641995}
10 files changed