Do not update last_activity_time_ in SpdySession::DoWriteComplete().

|last_activity_time_| is used to determine if a PING should be sent out
on an HTTP/2 connection.  The relevant measurement for this is the time
of the last read.  If |last_activity_time_| is also updated on writes,
then it is possible that a connection times out but SpdySession is not
sending any PING frames to check for this for a very long time.

This CL removes the |last_activity_time_| update from
SpdySession::DoWriteComplete(), and renames |last_activity_time_| to
|last_read_time_| to better express its function.

Bug: 784975
Change-Id: I821cc5145f44557dd3f694833314336bb21f9608
Reviewed-on: https://chromium-review.googlesource.com/779740


Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518290}