Attempt to fix issue 344300 by guarding against rootStyle being null.

From what I can tell this was introduced in issue 64293008 at Nov 29th 2013. Clusterfuzz also broke at that time with a similar stack trace. 

An alternatively fix could be to assert that rootStyle is never set to null in the constructors of CSSToLengthConversionData. However that seems to be a more involved fix and I will leave that for the owner of the code.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, timloh@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=344300

Review URL: https://codereview.chromium.org/170283003

git-svn-id: svn://svn.chromium.org/blink/trunk@167596 bbb929c8-8fbe-4397-9dbb-9b2b20218538