Initialize StyleSheetCollection::m_usesRemUnit.

The origial uninitialized variable wasn't harmful as it is
initialized later before use. This change is just for clarity.

BUG=none
TEST=none
R=tasak, hayato

Review URL: https://codereview.chromium.org/65043013

git-svn-id: svn://svn.chromium.org/blink/trunk@162091 bbb929c8-8fbe-4397-9dbb-9b2b20218538