Bug 1473453 - Part 1: a11y: assert `hitregion` property is always null or true. r=MarcoZ

Fix clang-tidy warning about bool pointers:

[misc-bool-pointer-implicit-conversion] accessible/generic/Accessible.cpp:644:9: warning: dubious check of 'bool *' against 'nullptr', did you mean to dereference it?

The `hitregion` property is always null or points to a true bool, set here:

https://searchfox.org/mozilla-central/rev/6ef785903fee6c0b16a1eab79d722373d940fd78/dom/canvas/CanvasRenderingContext2D.cpp#3936-3937

The `hitregion` property used to be an nsRect pointer:

https://searchfox.org/mozilla-central/diff/c797577640f306df87e8c32313c5b826d1e58a9b/accessible/src/generic/Accessible.cpp#928

MozReview-Commit-ID: HYlAnMyaitB

--HG--
extra : rebase_source : 2f8d9d151ebc382043c107517838ccdaa0ed7228

accessible/generic/Accessible.cpp

@@ -639,7 +639,10 @@ Accessible::RelativeBounds(nsIFrame** aBoundingFrame) const
 {
   nsIFrame* frame = GetFrame();
   if (frame && mContent) {
-    bool* hasHitRegionRect = static_cast<bool*>(mContent->GetProperty(nsGkAtoms::hitregion));
+    bool* pHasHitRegionRect = static_cast<bool*>(mContent->GetProperty(nsGkAtoms::hitregion));
+    MOZ_ASSERT(pHasHitRegionRect == nullptr ||
+               *pHasHitRegionRect, "hitregion property is always null or true");
+    bool hasHitRegionRect = pHasHitRegionRect != nullptr && *pHasHitRegionRect;
 
     if (hasHitRegionRect && mContent->IsElement()) {
       // This is for canvas fallback content