Increase maxBeaconTransmission default to 64k bytes.

The implementation should impose an upper bound on the amount of
per-document data that sendBeacon() is allowed to transmit.

Increase the default setting to a larger 64k (was: 16k), taking feedback from
some Google internal projects into account.

R=jochen@chromium.org
BUG=402991

Review URL: https://codereview.chromium.org/503843002

git-svn-id: svn://svn.chromium.org/blink/trunk@181229 bbb929c8-8fbe-4397-9dbb-9b2b20218538