Add signalSyncPoint to the WebGraphicsContext3D command buffer impls.

cc/output/gl_renderer_pixeltest.cc

Index: cc/output/gl_renderer_pixeltest.cc
diff --git a/cc/output/gl_renderer_pixeltest.cc b/cc/output/gl_renderer_pixeltest.cc
index 064b78bf27a2980d47aa8ca1256a81c7ff463f41..6ffa610de38d2681a9c7e2fdef2ebb0e5ee69736 100644
--- a/cc/output/gl_renderer_pixeltest.cc
+++ b/cc/output/gl_renderer_pixeltest.cc
@@ -6,6 +6,7 @@
 
 #include "cc/layers/append_quads_data.h"
 #include "cc/quads/draw_quad.h"
+#include "cc/resources/sync_point_helper.h"
 #include "cc/test/pixel_test.h"
 #include "third_party/skia/include/core/SkImageFilter.h"
 #include "third_party/skia/include/core/SkMatrix.h"
@@ -524,6 +525,20 @@ TEST_F(GLRendererPixelTest, AntiAliasing) {
       base::FilePath(FILE_PATH_LITERAL("anti_aliasing.png")),
       ExactPixelComparator(true)));
 }
+
+static void SyncPointCallback(int* callback_count) {
+  ++(*callback_count);
+}
+
+TEST_F(GLRendererPixelTest, SignalSyncPoint) {
+  int callback_count = 0;
+  scoped_ptr<SyncPointHelper> helper = SyncPointHelper::SignalSyncPoint(
+      output_surface_->context3d(),
+      output_surface_->context3d()->insertSyncPoint(),
+      base::Bind(&SyncPointCallback, &callback_count));
+  output_surface_->context3d()->finish();
+  EXPECT_EQ(1, callback_count);
+}
 #endif
 
 }  // namespace