Fix data races in QUIC end-to-end-tests.

Provide notifications for pausing/resuming the QuicServer, as
well as handshake confirmation in ServerThread to avoid data races.

Merge internal change: 56303744

Change end-to-end tests from using racy/too small packet size
to instead use large requests.

Merge internal change: 56174630

BUG=313733

Review URL: https://codereview.chromium.org/62203006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233958 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233958