Change GetTooltipText() to return the value directly

Instead of using an output parameter of pointer type, this CL changes
GetTooltipText() function to directly return the tooltip's value.

There is one case where the return value is used by ChromeOS
ShelfButton to indicate whether the tooltip should not be drawn by View.
We added one API for such case.

BUG=155933

Change-Id: Ie190c04c26b5d004857e9441994f12ebe84124a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516725
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641255}
57 files changed