Fix performance regression and bug with hide_scrollbars setting.

FrameView and PaintLayerScrollableArea may perform more work than
necessary if hide_scrollbars is true (e.g. trigger unnecessary
layouts), because they think that scrollbar existence has changed
even if it didn't.

For FrameView, we can avoid this by enforcing hide_scrollbars
in computeScrollbarExistence instead. For PLSA, we check for
hide_scrollbars in updateAfterLayout to determine if the
scrollbar existence will change.

Also fixes another potential bug in PLSA, which may incorrectly
assume existence of scrollbars if hide_scrollbars is true.

BUG=652317,639806
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2394163002
Cr-Commit-Position: refs/heads/master@{#423671}

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

@@ -3451,9 +3451,6 @@ void FrameView::addChild(Widget* child) {
 }
 
 void FrameView::setHasHorizontalScrollbar(bool hasBar) {
-  if (m_frame->settings() && m_frame->settings()->hideScrollbars())
-    hasBar = false;
-
   if (hasBar == !!m_horizontalScrollbar)
     return;
 
@@ -3477,9 +3474,6 @@ void FrameView::setHasHorizontalScrollbar(bool hasBar) {
 }
 
 void FrameView::setHasVerticalScrollbar(bool hasBar) {
-  if (m_frame->settings() && m_frame->settings()->hideScrollbars())
-    hasBar = false;
-
   if (hasBar == !!m_verticalScrollbar)
     return;
 
@@ -3691,6 +3685,12 @@ void FrameView::computeScrollbarExistence(
     bool& newHasVerticalScrollbar,
     const IntSize& docSize,
     ComputeScrollbarExistenceOption option) const {
+  if (m_frame->settings() && m_frame->settings()->hideScrollbars()) {
+    newHasHorizontalScrollbar = false;
+    newHasVerticalScrollbar = false;
+    return;
+  }
+
   bool hasHorizontalScrollbar = m_horizontalScrollbar;
   bool hasVerticalScrollbar = m_verticalScrollbar;
 

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

@@ -691,14 +691,20 @@ void PaintLayerScrollableArea::updateAfterLayout() {
 
   // We need to layout again if scrollbars are added or removed by
   // overflow:auto, or by changing between native and custom.
+  DCHECK(box().frame()->settings());
   bool horizontalScrollbarShouldChange =
-      (box().hasAutoHorizontalScrollbar() &&
-       (hasHorizontalScrollbar() != shouldHaveAutoHorizontalScrollbar)) ||
-      (box().style()->overflowX() == OverflowScroll && !horizontalScrollbar());
+      ((box().hasAutoHorizontalScrollbar() &&
+        (hasHorizontalScrollbar() != shouldHaveAutoHorizontalScrollbar)) ||
+       (box().style()->overflowX() == OverflowScroll &&
+        !horizontalScrollbar())) &&
+      (!box().frame()->settings()->hideScrollbars() ||
+       hasHorizontalScrollbar());
   bool verticalScrollbarShouldChange =
-      (box().hasAutoVerticalScrollbar() &&
-       (hasVerticalScrollbar() != shouldHaveAutoVerticalScrollbar)) ||
-      (box().style()->overflowY() == OverflowScroll && !verticalScrollbar());
+      ((box().hasAutoVerticalScrollbar() &&
+        (hasVerticalScrollbar() != shouldHaveAutoVerticalScrollbar)) ||
+       (box().style()->overflowY() == OverflowScroll &&
+        !verticalScrollbar())) &&
+      (!box().frame()->settings()->hideScrollbars() || hasVerticalScrollbar());
   bool scrollbarsWillChange =
       !scrollbarsAreFrozen && !visualViewportSuppliesScrollbars() &&
       (horizontalScrollbarShouldChange || verticalScrollbarShouldChange);
@@ -938,15 +944,13 @@ void PaintLayerScrollableArea::updateAfterStyleChange(
 
   // With overflow: scroll, scrollbars are always visible but may be disabled.
   // When switching to another value, we need to re-enable them (see bug 11985).
-  if (needsHorizontalScrollbar && oldStyle &&
+  if (hasHorizontalScrollbar() && oldStyle &&
       oldStyle->overflowX() == OverflowScroll && overflowX != OverflowScroll) {
-    ASSERT(hasHorizontalScrollbar());
     horizontalScrollbar()->setEnabled(true);
   }
 
-  if (needsVerticalScrollbar && oldStyle &&
+  if (hasVerticalScrollbar() && oldStyle &&
       oldStyle->overflowY() == OverflowScroll && overflowY != OverflowScroll) {
-    ASSERT(hasVerticalScrollbar());
     verticalScrollbar()->setEnabled(true);
   }