Reorder flag check for kMaxOverlapBoundsForFixed

Evaluating if the feature is enabled is more costly then evaluating if
it is fixed to the viewport. Swapping this order nets at 15% improvement
according to a linux-perf try job for the blink_perf.paint
fixed-and-many-layers-scroll perf test.

There is no functionality change with this modification.

Bug: 1083715
Change-Id: I0573dd56a9ac944517b952ec7b7e6d8534d478bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210780Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Sam Fortiner <samfort@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#771117}

third_party/blink/renderer/core/paint/paint_layer.cc

@@ -2597,9 +2597,9 @@ PhysicalRect PaintLayer::BoundingBoxForCompositingOverlapTest() const {
         style.BackdropFilter().MapRect(FloatRect(bounding_box)));
   }
 
-  if (base::FeatureList::IsEnabled(features::kMaxOverlapBoundsForFixed) &&
-      !bounding_box.IsEmpty()) {
-    if (FixedToViewport()) {
+  if (FixedToViewport()) {
+    if (base::FeatureList::IsEnabled(features::kMaxOverlapBoundsForFixed) &&
+        !bounding_box.IsEmpty()) {
       DCHECK_EQ(style.GetPosition(), EPosition::kFixed);
       // Note that we only expand the bounding box for overlap testing when the
       // fixed's containing block is the viewport. This keeps us from expanding