Remove mask field from HashTable.h and other cleanup.

There's no need to have the mask field, it can be calculated from the size,
which is actually slightly faster and saves a little memory. The stats counting
doesn't actually compile for me, but it didn't before either.

R=ager@chromium.org, mikhail.pozdnyakov@intel.com, wibling@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/268693003

git-svn-id: svn://svn.chromium.org/blink/trunk@173088 bbb929c8-8fbe-4397-9dbb-9b2b20218538