Bug 1590582 - Don't leave HitTestResult::mFixedPosSides uninitialized. r=tnikkel

Likewise for APZCTreeManager::mFixedPosSidesForInputBlock.

Differential Revision: https://phabricator.services.mozilla.com/D50360

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

gfx/layers/apz/src/APZCTreeManager.h

@@ -566,7 +566,7 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
     HitTestingTreeNodeAutoLock mScrollbarNode;
     // If content that is fixed to the root-content APZC was hit,
     // the sides of the viewport to which the content is fixed.
-    SideBits mFixedPosSides;
+    SideBits mFixedPosSides = eSideBitsNone;
 
     HitTestResult() = default;
     // Make it move-only.
@@ -865,7 +865,7 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    * used to offset event coordinates accordingly.
    * This should be in sync with mApzcForInputBlock.
    */
-  SideBits mFixedPosSidesForInputBlock;
+  SideBits mFixedPosSidesForInputBlock = eSideBitsNone;
   /* Sometimes we want to ignore all touches except one. In such cases, this
    * is set to the identifier of the touch we are not ignoring; in other cases,
    * this is set to -1.