[RootLayerScrolls] Don't double-adjust scroll offset for page zoom

The page zoom factor will propagate down to the LayoutView's style,
and the scroll offset will be adjusted in LayoutBox::StyleDidChange.

I'm not sure why the affected test isn't in FlagExpectations...

BUG=711468
R=skobes@chromium.org,pdr@chromium.org
TEST=cssom/elementfrompoint-scrolloffset.html

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_root_layer_scrolls
Change-Id: I87c8fc3334244b17b41fd7e185822b0f35a763bd
Reviewed-on: https://chromium-review.googlesource.com/775895Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517381}

third_party/WebKit/Source/core/frame/LocalFrame.cpp

@@ -585,7 +585,8 @@ void LocalFrame::SetPageAndTextZoomFactors(float page_zoom_factor,
       return;
   }
 
-  if (page_zoom_factor_ != page_zoom_factor) {
+  if (page_zoom_factor_ != page_zoom_factor &&
+      !RuntimeEnabledFeatures::RootLayerScrollingEnabled()) {
     if (LocalFrameView* view = this->View()) {
       // Update the scroll position when doing a full page zoom, so the content
       // stays in relatively the same position.