Bug 1662836 - Expose detailed initialization failure reason for WebRender. r=kvark

We don't know why we see initialization failures in the telemetry which
makes it hard to investigate why users aren't getting WebRender and
instead fallback to basic. Let's expose the detailed error message
WebRender already generates and puts in the critical log.

Differential Revision: https://phabricator.services.mozilla.com/D89185

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -58,7 +58,7 @@ KnowsCompositor* WebRenderLayerManager::AsKnowsCompositor() { return mWrChild; }
 
 bool WebRenderLayerManager::Initialize(
     PCompositorBridgeChild* aCBChild, wr::PipelineId aLayersId,
-    TextureFactoryIdentifier* aTextureFactoryIdentifier) {
+    TextureFactoryIdentifier* aTextureFactoryIdentifier, nsCString& aError) {
   MOZ_ASSERT(mWrChild == nullptr);
   MOZ_ASSERT(aTextureFactoryIdentifier);
 
@@ -71,13 +71,14 @@ bool WebRenderLayerManager::Initialize(
     // reinitialization. We can expect to be notified again to reinitialize
     // (which may or may not be using WebRender).
     gfxCriticalNote << "Failed to create WebRenderBridgeChild.";
+    aError.AssignLiteral("FEATURE_FAILURE_WEBRENDER_INITIALIZE_IPDL");
     return false;
   }
 
   TextureFactoryIdentifier textureFactoryIdentifier;
   wr::MaybeIdNamespace idNamespace;
   // Sync ipc
-  bridge->SendEnsureConnected(&textureFactoryIdentifier, &idNamespace);
+  bridge->SendEnsureConnected(&textureFactoryIdentifier, &idNamespace, &aError);
   if (textureFactoryIdentifier.mParentBackend == LayersBackend::LAYERS_NONE ||
       idNamespace.isNothing()) {
     gfxCriticalNote << "Failed to connect WebRenderBridgeChild.";