Bug 1577258 - early-return for noop resizes. r=botond

This avoids doing wasted work and sending spurious resize
events if this case would be hit.

In practice, it cannot be hit yet, I think, because
callers do check for this and bail out earlier. But
there's no assertion to that respect so this shouldn't
hurt.

Differential Revision: https://phabricator.services.mozilla.com/D43798

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

layout/base/PresShell.cpp

@@ -1863,6 +1863,10 @@ nsresult PresShell::ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight,
                                                ResizeReflowOptions aOptions) {
   MOZ_ASSERT(!mIsReflowing, "Shouldn't be in reflow here!");
 
+  if (aWidth == aOldWidth && aHeight == aOldHeight) {
+    return NS_OK;
+  }
+
   nsIFrame* rootFrame = mFrameConstructor->GetRootFrame();
   if (!rootFrame) {
     // If we don't have a root frame yet, that means we haven't had our initial