Re-add scrolling of the root frame for plugin windowRects.

This was mistakenly removed in https://codereview.chromium.org/1510083002.
https://codereview.chromium.org/1508363002 makes this clear also.

BUG=555381
TBR=wangxianzhu@chromium.org

Review URL: https://codereview.chromium.org/1506013006

Cr-Commit-Position: refs/heads/master@{#364447}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1302,7 +1302,9 @@ crbug.com/564109 [ Win7 XP ]  http/tests/webfont/font-display.html [ Pass Timeou
 crbug.com/558574 [ Win7 ] fast/forms/text/text-font-height-mismatch.html [ Failure ]
 crbug.com/558574 [ Win7 ] fast/text/emphasis.html [ Failure ]
 
-crbug.com/561595 [ XP ] plugins/webview-plugin-scroll.html [ Failure ]
+crbug.com/555381 plugins/webview-plugin-scroll.html [ NeedsRebaseline ]
+
+#crbug.com/561595 [ XP ] plugins/webview-plugin-scroll.html [ Failure ]
 crbug.com/561595 [ XP ] plugins/webview-plugin-nested-iframe-scroll.html [ Failure ]
 
 crbug.com/568157 [ XP ] virtual/syncpaint/fast/repaint/details-open-repaint.html [ Failure ]

third_party/WebKit/Source/web/WebPluginContainerImpl.cpp

@@ -951,9 +951,12 @@ void WebPluginContainerImpl::computeClipRectsForPlugin(
     LayoutRect unclippedAbsoluteRect(frameRectInOwnerElementSpace);
     box->mapRectToPaintInvalidationBacking(rootView, unclippedAbsoluteRect, nullptr);
 
-    // The frameRect is already in absolute space, except for scrolling of the root frame.
+    // The frameRect is already in absolute space of the local frame to the plugin.
     windowRect = frameRect();
+    // Map up to the root frame.
     windowRect = enclosingIntRect(m_element->document().view()->layoutView()->localToAbsoluteQuad(FloatQuad(FloatRect(frameRect())), TraverseDocumentBoundaries).boundingBox());
+    // Finally, adjust for scrolling of the root frame, which the above does not take into account.
+    windowRect.moveBy(roundedIntPoint(-rootView->viewRect().location()));
 
     clippedLocalRect = enclosingIntRect(unclippedAbsoluteRect);
     unclippedIntLocalRect = clippedLocalRect;