Bug 888781 part 1 - Use cached default scale in GetDefaultScaleInternal of Windows. r=jimm

MozReview-Commit-ID: Ebm7h3h2mrg

--HG--
extra : rebase_source : e37ddbfca0eee4a7ba04121709f93048b0aab899

widget/windows/nsWindow.cpp

@@ -1096,7 +1096,7 @@ double nsWindow::GetDefaultScaleInternal()
   if (mDefaultScale <= 0.0) {
     mDefaultScale = WinUtils::LogToPhysFactor(mWnd);
   }
-  return WinUtils::LogToPhysFactor(mWnd);
+  return mDefaultScale;
 }
 
 int32_t nsWindow::LogToPhys(double aValue)