Implements the ACK flag for SPDY4/HTTP2 PING frames

This lands server change 61378132 by birenroy.

Also update SpdySession, BufferedSpdyFramer, etc to accept and pass
the |is_ack| flag.

BUG=345769

Review URL: https://codereview.chromium.org/169283012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253088 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253088
Remove obsolete gin::IsolateHolder ctor

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/173843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253089 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253089