Refactor string classes and utility functions in net/http2.

Refactor string classes and utility functions:
* std::string to Http2String,
* base::StringPiece to Http2StringPiece,
* StringPiece::AppendToString and StringPiece::append to Http2StrAppend
* base::StringPrintf to Http2StringPrintf,
* implement Http2StrCat
in net/http2/platform, just like it has been done in net/spdy/platform.

This lands server side CL 161074972 by bnc.

BUG=488484

Change-Id: I893bf6c0b97eeea92aa06d3503edea85a9ce302c
Reviewed-on: https://chromium-review.googlesource.com/561756
Reviewed-by: Helen Li <[email protected]>
Commit-Queue: Bence Béky <[email protected]>
Cr-Commit-Position: refs/heads/master@{#485823}
92 files changed