Added missing mouse back/forward button plumbing in Blink.

Also renamed the enum labels for these buttons from X1/X2 to
more intuitive back/forward.

We will need more OS-dependent work in future CLs. E.g. in
Linux, back/forward buttons still seems "muted".

BUG=567771

Review-Url: https://codereview.chromium.org/2763313002
Cr-Commit-Position: refs/heads/master@{#461181}
16 files changed