Do not trigger compositing update on selection gap update for SPv2

Doing so results in assertion failure because the document lifecycle does not
allow rewinding from InPaintForSlimmingPaintV2 back to LayoutClean.

Skipping compositing update should be fine since SPv2 doesn't rely on old world
compositing anyway.

BUG=524879

Review URL: https://codereview.chromium.org/1312363003

git-svn-id: svn://svn.chromium.org/blink/trunk@201244 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.cpp

@@ -1942,6 +1942,9 @@ DeprecatedPaintLayer* DeprecatedPaintLayer::hitTestChildren(ChildrenIteration ch
 
 void DeprecatedPaintLayer::blockSelectionGapsBoundsChanged()
 {
+    if (RuntimeEnabledFeatures::slimmingPaintV2Enabled())
+        return;
+
     setNeedsCompositingInputsUpdate();
 }