Use an unsigned of flags in ElementRareData

Instead of exposing tons of getters and setters on Element we can use an
enum of flags and treat the bits inside ElementRareData as just a flag
field like we do in Node. This removes a lot of boilerplate code and also
makes the check for style sharing a lot simpler since now we can just
OR the flags that disable sharing together. That's good since that check
is super hot in profiles.

This also lets us make all the other "flag reading" methods into simple
inline getters that call hasElementFlag() which early outs on not having
any rare data.

Review URL: https://codereview.chromium.org/202633006

git-svn-id: svn://svn.chromium.org/blink/trunk@169426 bbb929c8-8fbe-4397-9dbb-9b2b20218538