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If the stream ID space is exhausted, SpdySession must allow active
streams to complete, but prevent new streams from being created.
Change SpdySession to begin going away in this case, and add an explicit
test on ID exhaustion.

Also allow stream IDs to use the full 31-bit space available.

BUG=373858

Review URL: https://codereview.chromium.org/287063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271175 0039d316-1c4b-4281-b951-d872f2087c98