[TextAutosizer] Fix a crash where fingerprints were not reset

If autosizing was disabled after fingerprints were added, we could fail
to update the fingerprint mapper when a RenderBlock was destroyed.
This was a common crash scenario when enabling and disabling mobile
emulation in devtools.

The fix is to simply check if fingerprints exist when destroying
blocks so that they are properly removed from the fingerprint maps.
The performance hit of this additional check shouldn't be too bad as
it ends up being a check of one bit.

This patch does not remove several speculative fixes for the same
bug because it is not clear that all crashes have the same root cause.
A followup patch will remove these speculative fixes.

R=skobes
BUG=369485
BUG=415607

Review URL: https://codereview.chromium.org/597463003

git-svn-id: svn://svn.chromium.org/blink/trunk@182517 bbb929c8-8fbe-4397-9dbb-9b2b20218538