[FastTextAutosizing] Remove FastTextAutosizer::enabled

FastTextAutosizer::enabled() is hot on profiles as it's called multiple
times per block. The places where the text autosizing setting can
change are well defined and all go through updatePageInfo() so this
patch removes enabled() entirely, replacing it with a check of
m_pageInfo.m_settingEnabled which is updated in updatePageInfo().

This patch is a followup to
https://src.chromium.org/viewvc/blink?view=rev&revision=173020

No new tests as this is just a refactoring.

BUG=367864

Review URL: https://codereview.chromium.org/265713002

git-svn-id: svn://svn.chromium.org/blink/trunk@173042 bbb929c8-8fbe-4397-9dbb-9b2b20218538