Collect active stylesheets and and apply asynchronously.

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

Index: third_party/WebKit/Source/core/frame/LocalFrame.cpp
diff --git a/third_party/WebKit/Source/core/frame/LocalFrame.cpp b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
index fce55762b41d60740ebe853b59143a1b8a45b3a7..35e3a704187450276f3df95c0469eb2e1ed6749d 100644
--- a/third_party/WebKit/Source/core/frame/LocalFrame.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
@@ -695,6 +695,9 @@ void LocalFrame::setPageAndTextZoomFactors(float pageZoomFactor,
 
 void LocalFrame::deviceScaleFactorChanged() {
   document()->mediaQueryAffectingValueChanged();
+  document()->setNeedsStyleRecalc(

[esprehn, 2016/12/09 01:36:21]

Did this cause a full document recalc before? I didn't think we used to do a
full recalc on every zoom before. I think we're going to use dsf for zoom soon
too which means this case will need to be fast.

[rune, 2016/12/09 08:55:50]

Yes, mediaQueryAffectingValueChanged called resolverChanged(FullStyleUpdate)
which re-collects all stylesheets in all scopes and marks Document
SubtreeStyleChange in the StyleSheetChange analysis in the
StyleSheetCollections. Without this change,
fast/events/mouse-cursor-image-set.html will fail on Mac. That test is expecting
a change in which image from an image set is picked for a given zoom factor.

testRunner.setBackingScaleFactor() seems to use DSF on Mac and
setPageAndTextZoomFactors() on other platforms.

This can probably be improved, but that could be a separate task. This should be
no perf regressions with this CL.

+      SubtreeStyleChange,
+      StyleChangeReasonForTracing::create(StyleChangeReason::Zoom));
   for (Frame* child = tree().firstChild(); child;
        child = child->tree().nextSibling()) {
     if (child->isLocalFrame())