Remove PaintInfo::PaintContainer()

Its usages were:
1. ScopedPaintState checks it to see if it needs
   PaintOffsetTranslation update. Now checks if the current box has a
   self-painting layer.
2. BoxDecorationData checks it to see whether to ignore background.
   In CompositeAfterPaint, the ShouldSkipBackground flag is set only
   temporarily, so we don't need to check PaintContainer().

Bug: 1277748
Change-Id: I8c55212fe073bdece24cfcbf14456d03e8a769b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3365959


Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#956755}