Content-sized resolution should only happen on content-sized tracks

The current code would always try to run the content sized
algorithm regardless of whether this was needed. On top of
it, it would iterate over ALL grid items, even if only a
small set of the tracks were content-sized.

This change stores of which tracks are content-sized and
use the internal grid to only iterate over these tracks.

On a Z620, this speeds up
PerformanceTests/Layout/fixed-grid-lots-of-data.html from
about 550 runs / sec to 830 runs / sec.

BUG=273238

Review URL: https://chromiumcodereview.appspot.com/22867006

git-svn-id: svn://svn.chromium.org/blink/trunk@156168 bbb929c8-8fbe-4397-9dbb-9b2b20218538