[devtools] Add a command to emulate the default background color.

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 6556ad47ace0066df479fab61b784c305194355b..4372e4a3c42254acddaa7fbbbc83dfcffe800744 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -196,6 +196,7 @@ FrameView::FrameView(LocalFrame& frame)
       m_needsScrollbarsUpdate(false),
       m_suppressAdjustViewSize(false),
       m_allowsLayoutInvalidationAfterLayoutClean(true),
+      m_needsOpaquenessUpdate(false),
       m_mainThreadScrollingReasons(0),
       m_mainThreadScrollingReasonsCounter(
           MainThreadScrollingReason::kMainThreadScrollingReasonCount,
@@ -2308,6 +2309,16 @@ Color FrameView::baseBackgroundColor() const {
 
 void FrameView::setBaseBackgroundColor(const Color& backgroundColor) {
   m_baseBackgroundColor = backgroundColor;
+  m_needsOpaquenessUpdate = true;
+  recalculateScrollbarOverlayColorTheme(documentBackgroundColor());
+
+  if (!shouldThrottleRendering())
+    page()->animator().scheduleVisualUpdate(m_frame.get());
+}
+
+void FrameView::updateOpaquenessIfNeeded() {
+  if (!m_needsOpaquenessUpdate)
+    return;
 
   if (!layoutViewItem().isNull() &&
       layoutViewItem().layer()->hasCompositedLayerMapping()) {
@@ -2317,10 +2328,7 @@ void FrameView::setBaseBackgroundColor(const Color& backgroundColor) {
     if (compositedLayerMapping->mainGraphicsLayer())
       compositedLayerMapping->mainGraphicsLayer()->setNeedsDisplay();
   }
-  recalculateScrollbarOverlayColorTheme(documentBackgroundColor());
-
-  if (!shouldThrottleRendering())
-    page()->animator().scheduleVisualUpdate(m_frame.get());
+  m_needsOpaquenessUpdate = false;
 }
 
 void FrameView::updateBackgroundRecursively(const Color& backgroundColor,
@@ -2957,6 +2965,8 @@ void FrameView::updateLifecyclePhasesInternal(
         });
       }
 
+      updateOpaquenessIfNeeded();
+
       scrollContentsIfNeededRecursive();
       DCHECK(RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled() ||
              lifecycle().state() >= DocumentLifecycle::CompositingClean);