Remove canSuspend() and ReasonForSuspension which are no longer used.

Cache is gone.

canSuspend() is no longer used by any component.

Only WillDeferLoading in ReasonForSuspension is used, so it's ok to eliminate
reason argument from suspend().

BUG=none

Review URL: https://codereview.chromium.org/27015002

git-svn-id: svn://svn.chromium.org/blink/trunk@159535 bbb929c8-8fbe-4397-9dbb-9b2b20218538