cc: Allow output readback/copy requests to specify a sub-rect to copy.

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index 2a27ff7810416861d3d38af9db571d95038265bf..3527ed1138a215662797decb54b99b96b4d36320 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -1914,8 +1914,13 @@ void GLRenderer::EnsureScissorTestDisabled() {
 void GLRenderer::CopyCurrentRenderPassToBitmap(
     DrawingFrame* frame,
     scoped_ptr<CopyOutputRequest> request) {
-  GetFramebufferPixelsAsync(frame->current_render_pass->output_rect,
-                            request.Pass());
+  gfx::Rect copy_rect = frame->current_render_pass->output_rect;
+  if (request->has_area()) {
+    // Intersect with the request's area, positioned with its origin at the
+    // origin of the full copy_rect.
+    copy_rect.Intersect(request->area() - copy_rect.OffsetFromOrigin());
+  }
+  GetFramebufferPixelsAsync(copy_rect, request.Pass());
 }
 
 void GLRenderer::ToGLMatrix(float* gl_matrix, const gfx::Transform& transform) {