Bug 1568673 part 2: Add a few more assertions to validate that mobile viewport zoom factors are positive. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D39244

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

layout/base/MobileViewportManager.cpp

@@ -180,6 +180,15 @@ CSSToScreenScale MobileViewportManager::ClampZoom(
     zoom = aViewportInfo.GetMaxZoom();
     MVM_LOG("%p: Clamped to %f\n", this, zoom.scale);
   }
+
+  // Non-positive zoom factors can produce NaN or negative viewport sizes,
+  // so we better be sure we've got a positive zoom factor. Just for good
+  // measure, we check our min/max as well as the final clamped value.
+  MOZ_ASSERT(aViewportInfo.GetMinZoom() > CSSToScreenScale(0.0f),
+             "zoom factor must be positive");
+  MOZ_ASSERT(aViewportInfo.GetMaxZoom() > CSSToScreenScale(0.0f),
+             "zoom factor must be positive");
+  MOZ_ASSERT(zoom > CSSToScreenScale(0.0f), "zoom factor must be positive");
   return zoom;
 }