Bug 1690925 - Don't animate the skeleton UI if we're in a fast startup r=mconley,emalysz

This avoids paying a cost for animating the skeleton UI for faster startups.
Slow startups don't seem to be affected, given the data available. This is
likely due to slow startups being dominated by IO.

Differential Revision: https://phabricator.services.mozilla.com/D104118

mozglue/misc/PreXULSkeletonUI.cpp

@@ -1150,6 +1150,21 @@ DWORD WINAPI AnimateSkeletonUI(void* aUnused) {
     return 0;
   }
 
+  // See the comments above the InterlockedIncrement calls below here - we
+  // atomically flip this up and down around sleep so the main thread doesn't
+  // have to wait for us if we're just sleeping.
+  if (InterlockedIncrement(&sAnimationControlFlag) != 1) {
+    return 0;
+  }
+  // Sleep for two seconds - startups faster than this don't really benefit
+  // from an animation, and we don't want to take away cycles from them.
+  // Startups longer than this, however, are more likely to be blocked on IO,
+  // and thus animating does not substantially impact startup times for them.
+  ::Sleep(2000);
+  if (InterlockedDecrement(&sAnimationControlFlag) != 0) {
+    return 0;
+  }
+
   // On each of the animated rects (which happen to all be placeholder UI
   // rects sharing the same color), we want to animate a gradient moving across
   // the screen from left to right. The gradient starts as the rect's color on,