Use children().size() where relevant in ui/views/.

This eliminates some uses of child_count().

Bug: 940135
Change-Id: I8720a5f2be0da68fb4c2b55e51cbebaa2cd95c45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536739
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645215}
17 files changed