Remove PaintLayer::MapRectInPaintInvalidationContainerToBacking()

It's dead code.

Change-Id: I8b566e38858aa66aaf3a5316771056f7d5a2c0cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577571
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653287}
5 files changed