Bug 1903141 Part 2 - Annotate EnsurePushedFloats() as MOZ_NONNULL_RETURN. r=dholbert

`EnsurePushedFloats()` either returns the existing pushed floats list or creates
a new empty one. Therefore, it can never return nullptr.

Differential Revision: https://phabricator.services.mozilla.com/D214045

layout/generic/nsBlockFrame.h

@@ -977,7 +977,7 @@ class nsBlockFrame : public nsContainerFrame {
 
   // Get the pushed floats list, or if there is not currently one,
   // make a new empty one.
-  nsFrameList* EnsurePushedFloats();
+  nsFrameList* EnsurePushedFloats() MOZ_NONNULL_RETURN;
 
   // Get the pushed float list and remove the property from this frame.
   //