Fixes for re-enabling more MSVC level 4 warnings: Source/core/xml/ edition

This contains fixes for the following sorts of issues:
* Assignment inside conditional

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/383083008

git-svn-id: svn://svn.chromium.org/blink/trunk@177980 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add UseCounter for flex items having intinsic aspect ratios

According to flexbox spec http://dev.w3.org/csswg/css-flexbox/#algo-main-item
we should take intrinsic aspect ratios into account during main-size calculation.
Currently none of the browser is taking aspect ratio into account while calculating
main-size of flexbox.

Adding a use counter to determine how common it is for a flex item
to have intrinsic aspect ratios.

BUG=249112

Review URL: https://codereview.chromium.org/381613002

git-svn-id: svn://svn.chromium.org/blink/trunk@177981 bbb929c8-8fbe-4397-9dbb-9b2b20218538