Improve code with early returns.

Improve code with early returns.

Improve SpdyHttpUtils::GetUrlFromHeaderBlock() implementation with early
returns.  Get rid of a number of temporary variables that were declared too far
from where they were first used.

Also remove SPDY2-specific code, as SPDY2 is no longer supported in Chromium.

Acknowledgements to Daniel Bratell for suggesting to use append instead of
inline operator+ that would create unnecessary temporary strings, see
https://crrev.com/965773002.

BUG=

Committed: https://crrev.com/f5a07dab231907ba19ada492ac852b3a7037190c
Cr-Commit-Position: refs/heads/master@{#319784}

[Bence, 2015-03-09 21:42:08]

bnc@chromium.org changed reviewers:
+ rch@chromium.org

[Bence, 2015-03-09 21:42:09]

Ryan, PTAL.  Thanks.

[Ryan Hamilton, 2015-03-09 22:53:01]

lgtm

[Bence, 2015-03-09 23:03:33]

The CQ bit was checked by bnc@chromium.org

[commit-bot: I haz the power, 2015-03-09 23:05:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/996483002/1

[commit-bot: I haz the power, 2015-03-10 00:04:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-10 00:04:43]

Patchset 1 (id:??) landed as
https://crrev.com/f5a07dab231907ba19ada492ac852b3a7037190c
Cr-Commit-Position: refs/heads/master@{#319784}