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The internal read buffer for upload is 14520 for QUIC, 2852 for
SPDY, and 16384 for normal stream. SPDY is too small compared
to normal stream and QUIC. This CL increases the read buffer to
16384 so it is comparable to the other two implementations.
This will make it easier for net embedders to configure uploads to
work well across different transports.

This CL additionally updates flow control tests so they take
into account the new buffer size when zeroing out window size.

BUG=606784

Review URL: https://codereview.chromium.org/1914663002

Cr-Commit-Position: refs/heads/master@{#389856}