[FastTextAutosizer] Optimize clusterHasEnoughTextToAutosize

This patch optimizes clusterHasEnoughTextToAutosize by returning early
if the text length is greater than a minimum ("minimumTextLengthToAutosize").

I've also optimized superclusterMultiplier to early-out of a for loop once a
candidate has been found with enough text to autosize.

Lastly, this patch has some refactoring for clarity. The unnecessary
anyClusterHasEnoughText variable on Supercluster has been removed.
textLength() has been merged into clusterHasEnoughTextToAutosize, and
a comment has been added describing why 4 lines of text is considered
sufficient.

No new tests as there's no change in behavior.

BUG=302005

Review URL: https://codereview.chromium.org/180243004

git-svn-id: svn://svn.chromium.org/blink/trunk@167982 bbb929c8-8fbe-4397-9dbb-9b2b20218538