cc: Don't discard framebuffer when using partial swaps

cc/debug/test_web_graphics_context_3d.h

Index: cc/debug/test_web_graphics_context_3d.h
diff --git a/cc/debug/test_web_graphics_context_3d.h b/cc/debug/test_web_graphics_context_3d.h
index 3cc7073a890493abd0cd2c8551f4ce215115280a..4db270c4dd669b54cf76ded62f92575cd876910c 100644
--- a/cc/debug/test_web_graphics_context_3d.h
+++ b/cc/debug/test_web_graphics_context_3d.h
@@ -190,6 +190,9 @@ class CC_EXPORT TestWebGraphicsContext3D : public FakeWebGraphicsContext3D {
   void set_have_post_sub_buffer(bool have) {
     test_capabilities_.post_sub_buffer = have;
   }
+  void set_have_discard_framebuffer(bool have) {
+    test_capabilities_.discard_framebuffer = have;
+  }
 
   // When this context is lost, all contexts in its share group are also lost.
   void add_share_group_context(WebKit::WebGraphicsContext3D* context3d) {