base: Cleanup class/struct forward declarations

cc/test/fake_raster_source.h

Index: cc/test/fake_raster_source.h
diff --git a/cc/test/fake_raster_source.h b/cc/test/fake_raster_source.h
index ac24f1b631d79fedd2778b2d8aace1a937b4c5f0..3ed2e61211fc232f62b39beb296398621069f908 100644
--- a/cc/test/fake_raster_source.h
+++ b/cc/test/fake_raster_source.h
@@ -12,6 +12,10 @@
 
 class SkCanvas;
 
+namespace base {
+class WaitableEvent;
+}
+
 namespace cc {
 
 class RecordingSource;