Fix leak in textToSheetCache.

The leak happens because we always add a value to the textToSheetCache
but in some cases the sheet is not cachable. That leads to a situation
where we have a string->null mapping. Mappings are only cleared when
the value dies and pulls itself out of the mapping. If the value is
null that entry will never be cleared out.

For oilpan we had to hack our way around this issue. Instead of the
hacks we should fix the issue.

R=erik.corry@gmail.com, haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/194933002

git-svn-id: svn://svn.chromium.org/blink/trunk@168924 bbb929c8-8fbe-4397-9dbb-9b2b20218538