Bug 1577236 - clang-10: Fix -Wimplicit-int-float-conversion warnings in dom/vr/ r=kip

Depends on D43781

Differential Revision: https://phabricator.services.mozilla.com/D43782

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

dom/vr/VRDisplay.cpp

@@ -743,7 +743,7 @@ void VRFrameInfo::Update(const gfx::VRDisplayInfo& aInfo,
      * has a base of 0, which is not necessarily true in all UA's.
      */
     mTimeStampOffset =
-        float(rand()) / RAND_MAX * 10000.0f + 1000.0f - aState.timestamp;
+        float(rand()) / float(RAND_MAX) * 10000.0f + 1000.0f - aState.timestamp;
   }
   mVRState.timestamp = aState.timestamp + mTimeStampOffset;