Bug 1495421 - Fix gfxVRExtenal::StartPresentation value reset on Android r=kip

WebVR broke in Firefox Reality after https://phabricator.services.mozilla.com/D3378 landed because deleting the watchdog block also deleted some value resets in the Android #ifdef

Differential Revision: https://phabricator.services.mozilla.com/D7282

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

gfx/vr/gfxVRExternal.cpp

@@ -136,6 +136,11 @@ VRDisplayExternal::StartPresentation()
   if (mDisplayInfo.mDisplayState.mReportsDroppedFrames) {
     mTelemetry.mLastDroppedFrameCount = mDisplayInfo.mDisplayState.mDroppedFrameCount;
   }
+
+#if defined(MOZ_WIDGET_ANDROID)
+  mLastSubmittedFrameId = 0;
+  mLastStartedFrame = 0;
+#endif
 }
 
 void