Build skia as a dll in windows multi-dll build.

skia/ext/platform_canvas.h

Index: skia/ext/platform_canvas.h
===================================================================
--- skia/ext/platform_canvas.h	(revision 78242)
+++ skia/ext/platform_canvas.h	(working copy)
@@ -16,7 +16,7 @@
 // This class is a specialization of the regular SkCanvas that is designed to
 // work with a PlatformDevice to manage platform-specific drawing. It allows
 // using both Skia operations and platform-specific operations.
-class PlatformCanvas : public SkCanvas {
+class SK_API PlatformCanvas : public SkCanvas {
  public:
   // If you use the version with no arguments, you MUST call initialize()
   PlatformCanvas();