Prevent font cache purge during width calculation

Calculating text width could use system fallback fonts which are not
retained by the font cache. We should prevent font cache purging to make
sure that the unretained SimpleFontData is alive during width
calculation.

BUG=366571

Review URL: https://codereview.chromium.org/257783002

git-svn-id: svn://svn.chromium.org/blink/trunk@173139 bbb929c8-8fbe-4397-9dbb-9b2b20218538