Bug 1574281. Allow PWebRenderBridge::Msg_GetSnapshot to happen earlier. r=mconley

The graphics sanity test code calls GetSnapshot early on (see bug 1573852). We
currently allow it later, but there's nothing that keeps it from happening
early so we should allow it.

Differential Revision: https://phabricator.services.mozilla.com/D42191

--HG--
extra : moz-landing-system : lando

browser/base/content/test/performance/browser_startup_syncIPC.js

@@ -93,6 +93,12 @@ const startupPhases = {
       ignoreIfUnused: true, // Only on Win10 64
       maxCount: 1,
     },
+    {
+      name: "PWebRenderBridge::Msg_GetSnapshot",
+      condition: WIN && WEBRENDER,
+      ignoreIfUnused: true, // Sometimes in the next phase on Windows10 QR
+      maxCount: 1,
+    },
     {
       name: "PCompositorBridge::Msg_WillClose",
       condition: WIN,