Bug 1124029 - Fix 64bit shift warning in imgFrame. r=tn

--HG--
extra : rebase_source : 9d59de8b12bd00fc9ed6169b6bb60070f4868670
extra : source : cd0da193d6229fe4e7c57950aa42929164eb1f7d

image/src/imgFrame.h

@@ -295,7 +295,7 @@ private: // methods
 
   uint32_t PaletteDataLength() const
   {
-    return mPaletteDepth ? (1 << mPaletteDepth) * sizeof(uint32_t)
+    return mPaletteDepth ? (size_t(1) << mPaletteDepth) * sizeof(uint32_t)
                          : 0;
   }