In cases where a mouse click that starts a scrollbar drag is also what
layerizes the scroll frame, the StartAsyncScrollbarDrag message needs to
arrive after the layer transaction. This patch ensures it does.
MozReview-Commit-ID: A02qRb6yWxg
The bulk of this commit was generated by running:
run-clang-tidy.py \
-checks='-*,llvm-namespace-comment' \
-header-filter=^/.../mozilla-central/.* \
-fix