Fix first paint tracking for SPv2

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

Index: third_party/WebKit/Source/core/paint/ViewPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/ViewPainter.cpp b/third_party/WebKit/Source/core/paint/ViewPainter.cpp
index ac91a1edeb46d394339a162e1c8b620093905389..2d3ac26d80527cf1138313cacbc200fadde28876 100644
--- a/third_party/WebKit/Source/core/paint/ViewPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/ViewPainter.cpp
@@ -167,12 +167,10 @@ void ViewPainter::paintBoxDecorationBackground(const PaintInfo& paintInfo) {
       shouldDrawBackgroundInSeparateBuffer
           ? rootBackgroundColor
           : baseBackgroundColor.blend(rootBackgroundColor);
-  if (combinedBackgroundColor != frameView.baseBackgroundColor() &&
-      !context.getPaintController().nonDefaultBackgroundColorPainted()) {
-    TRACE_EVENT_INSTANT0("blink.user_timing", "paintNonDefaultBackgroundColor",
-                         TRACE_EVENT_SCOPE_GLOBAL);

[Xianzhu, 2017/01/10 23:29:01]

@panicker: I'm not sure if the above user_timing event is used and how it's
used. If we do need it, I can add back
PaintController::(setN|n)onDefaultBackgroundColorPainted(). Or do we just need
the event of first-painted?

[panicker, 2017/01/11 00:12:07]

It's fine to remove that trace event (was primarily used by me).

[panicker, 2017/01/11 00:17:25]

Sorry I misunderstood your question I think, we definitely want to keep the
logic that first-paint should only fire non-default background has painted (for
intance it should not fire on paint of white / default browser background
color).
So yes please keep the check for 
!context.getPaintController().nonDefaultBackgroundColorPainted()

[Xianzhu, 2017/01/11 00:23:41]

I think this change doesn't break that. The new code here will set firstPainted
to true when a non-default background is painted.
PaintController::processNewItem() will also set firstPainted to true when a
non-viewport-background is painted. They combined replace the original logic
here and the logic in GraphicsLayer checking the first paint.

[panicker, 2017/01/11 21:13:34]

Ok makes sense, thanks.

-    context.getPaintController().setNonDefaultBackgroundColorPainted();
-  }
+
+  if (combinedBackgroundColor != frameView.baseBackgroundColor())
+    context.getPaintController().setFirstPainted();
+
   if (combinedBackgroundColor.alpha()) {
     if (!combinedBackgroundColor.hasAlpha() &&
         RuntimeEnabledFeatures::slimmingPaintV2Enabled())