Adding backgroundColor to WebCompositionUnderline and using it for InlineTextBox drawing.

Source/core/rendering/RenderTheme.h

Index: Source/core/rendering/RenderTheme.h
diff --git a/Source/core/rendering/RenderTheme.h b/Source/core/rendering/RenderTheme.h
index a3ec3f66fed625cdb9eda8407b8b0675a0cc0a37..0ec13a3b4f2fad64f376efa1bacec480a2f29968 100644
--- a/Source/core/rendering/RenderTheme.h
+++ b/Source/core/rendering/RenderTheme.h
@@ -141,6 +141,7 @@ public:
     void setCustomFocusRingColor(const Color&);
     static Color tapHighlightColor();
     virtual Color platformTapHighlightColor() const { return RenderTheme::defaultTapHighlightColor; }
+    virtual Color platformDefaultCompositionBackgroundColor() const { return defaultCompositionBackgroundColor; }
     virtual void platformColorsDidChange();
 
     virtual double caretBlinkInterval() const { return 0.5; }
@@ -306,6 +307,8 @@ private:
     // making it more transparent than its alpha value indicates.
     static const RGBA32 defaultTapHighlightColor = 0x66000000;
 
+    static const RGBA32 defaultCompositionBackgroundColor = 0xFFFFDD55;
+
 #if USE(NEW_THEME)
     Theme* m_platformTheme; // The platform-specific theme.
 #endif