Initialize bitmap to (0, 0, 0, 0) for CanvasPaintT<T> instances.

skia/ext/canvas_paint_x.h

Index: skia/ext/canvas_paint_x.h
===================================================================
--- skia/ext/canvas_paint_x.h	(revision 128476)
+++ skia/ext/canvas_paint_x.h	(working copy)
@@ -17,6 +17,7 @@
 // A class designed to translate skia painting into a region in a Wayland window
 // surface. On construction, it will set up a context for painting into, and on
 // destruction, it will commit it to the Wayland window surface.
+// Note: The created context is always inialized to (0, 0, 0, 0).
 template <class T>
 class CanvasPaintT : public T {
  public:
@@ -88,6 +89,8 @@
       // Cause a deliberate crash;
       CHECK(false);
     }
+    // No need to clear the canvas, because cairo automatically performs the
+    // clear.
 
     // Need to translate so that the dirty region appears at the origin of the
     // surface.