Only not take zOffset into account during hit-testing when child layers are in the same 3d rendering context

Currently, hit-testing computes zOffset of layers with transformations. When
two layers overlap, it returns the layer with the highest zOffset.

Hit-testing should behave this way only when preserve3d is true.

BUG=126479,310464

Review URL: https://codereview.chromium.org/79943002

git-svn-id: svn://svn.chromium.org/blink/trunk@177685 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Avoid unnecessary scrollbars for sub-pixel layers

Change RenderLayerScrollableArea to compare the pixel snapped scroll
and client sizes. This mirrors the logic for RenderBlock and avoids
showing scrollbars and enabling scrolling in cases where the sub-pixel
values are different but snap to the same pixel value.

TEST=fast/sub-pixel/flexbox-unnecessary-scrollbar.htm
BUG=390407
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/372863002

git-svn-id: svn://svn.chromium.org/blink/trunk@177686 bbb929c8-8fbe-4397-9dbb-9b2b20218538