Revert r174230 - "Extend WidthCache to include glyph overflow bounds"

Several devtools layout tests began timing out on 10.6 (dbg) in
the first build that contained this change. This is a speculative
revert to see if things start working again, though it seems unlikely.

TBR=eae@chromium.org
BUG=374961

Review URL: https://codereview.chromium.org/291033003

git-svn-id: svn://svn.chromium.org/blink/trunk@174318 bbb929c8-8fbe-4397-9dbb-9b2b20218538