Fix Preloader Meta Accept-CH bug

Fix Preloader Meta Accept-CH bug

The previous addition of HTMLPreloadScanner support for meta Accept-CH
tags [1] was buggy, and the tests were buggy as well and failed to catch that :(
This CL fixes that bug, by parsing the actual header value, rather than the header name (doh!).

[1] https://codereview.chromium.org/1121263002/

BUG=482840
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197470

[Yoav Weiss, 2015-06-19 10:58:24]

yoav@yoav.ws changed reviewers:
+ mkwst@chromium.org

[Yoav Weiss, 2015-06-19 10:58:24]

Hey Mike! Can you take a look?

[Mike West, 2015-06-19 12:16:08]

LGTM: Can you add some detail to the CL description explaining the bug that this
is fixing?

https://codereview.chromium.org/1189403002/diff/40001/Source/core/html/parser...
File Source/core/html/parser/HTMLPreloadScanner.cpp (right):

https://codereview.chromium.org/1189403002/diff/40001/Source/core/html/parser...
Source/core/html/parser/HTMLPreloadScanner.cpp:545:
handleAcceptClientHintsHeader(String(contentAttribute->value),
m_clientHintsPreferences, nullptr);
Heh. Your reviewer should totally have caught that.

[Yoav Weiss, 2015-06-19 13:35:06]

The CQ bit was checked by yoav@yoav.ws

[commit-bot: I haz the power, 2015-06-19 13:35:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1189403002/40001

[commit-bot: I haz the power, 2015-06-19 13:38:43]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197470