Bug 1251612 - Support the GetFrameUniformity API in content processes. r=botond,froydnj

This moves the IPC mechanism from PCompositorBridge to PLayerTransaction/
PWebRenderBridge, so that it can be used by content processes like the other
test APIs. It still only produces actual data for the layers backend; for
WR it will just return empty datasets.

Differential Revision: https://phabricator.services.mozilla.com/D86016

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -721,5 +721,9 @@ void WebRenderLayerManager::WrReleasedImages(
   mStateManager.WrReleasedImages(aPairs);
 }
 
+void WebRenderLayerManager::GetFrameUniformity(FrameUniformityData* aOutData) {
+  WrBridge()->SendGetFrameUniformity(aOutData);
+}
+
 }  // namespace layers
 }  // namespace mozilla