Eliminating FIXME from RenderFlexibleBox::mainAxisContentExtent

A FIXME was added in RenderFlexibleBox::mainAxisContentExtent as
a part of the patch (https://bugs.webkit.org/attachment.cgi?id=193373&action=diff) 
stating:

"Remove std::max once we enable saturated layout arithmetic."

Since as per BUG=229103 SATURATED_LAYOUT_ARITHMETIC has been enabled for the chromiuim port 
so we can remove this std::max which was added to handle overflow.

Review URL: https://codereview.chromium.org/502733002

git-svn-id: svn://svn.chromium.org/blink/trunk@180884 bbb929c8-8fbe-4397-9dbb-9b2b20218538