Fix HTMLToken::Attribute::value handling in AtomicHTMLToken

Fix HTMLToken::Attribute::value handling in AtomicHTMLToken

|HTMLToken::Attribute::value| should be used as |String(value)| or
|StringImpl::create8BitIfPossible(value)|.
- |value.data()| is nullptr for attributes with no values, and such |value|
  should be converted to the empty string (WTF::String or AtomicString),
  because the null string is used for absence of an attribute.
- |String(value)| and |StringImpl::create8BitIfPossible(value)| converts
  |value| with null |value.data()| into the empty string.

However, |AtomicString(value)| converts |value| with null |value.data()| into
the null string.
This is done in AtomicHTMLToken, so this CL fixes it.

Also, this CL
- Adds unit tests for attributes with empty value,
- Adds CORE_EXPORT to classes/function needed by the unit tests, and
- Creates AtomicHTMLToken.cpp and moves implementation of AtomicHTMLToken from
  HTMLTokenizer.cpp.

BUG=540546
Committed: https://crrev.com/37d6b63597f41322f6092405ae913b19a7e85686
Cr-Commit-Position: refs/heads/master@{#354704}

[hiroshige, 2015-10-07 08:52:18]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-07 08:52:59]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/1

[commit-bot: I haz the power, 2015-10-07 10:01:48]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-07 10:01:49]

Dry run: This issue passed the CQ dry run.

[Yoav Weiss, 2015-10-09 07:37:14]

This fix makes sense. Would it be possible to add tests that make sure the fix
sticks around?

[hiroshige, 2015-10-13 10:00:12]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-13 10:00:41]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/20001

[hiroshige, 2015-10-13 10:02:00]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-13 10:02:30]

Dry run: Try jobs failed on following builders:
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  mac_chromium_gn_rel on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_gn_r...)
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[commit-bot: I haz the power, 2015-10-13 10:02:33]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/40001

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

The CQ bit was unchecked by commit-bot@chromium.org

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

Dry run: Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)
  mac_chromium_gn_rel on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_gn_r...)

[hiroshige, 2015-10-13 10:07:07]

Patchset #2 (id:20001) has been deleted

[hiroshige, 2015-10-13 10:07:14]

Patchset #2 (id:40001) has been deleted

[hiroshige, 2015-10-13 10:07:20]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-13 10:07:37]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/60001

[commit-bot: I haz the power, 2015-10-13 10:22:06]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-13 10:22:07]

Dry run: Try jobs failed on following builders:
  android_clang_dbg_recipe on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_clang_d...)

[hiroshige, 2015-10-13 11:07:16]

Patchset #4 (id:100001) has been deleted

[hiroshige, 2015-10-13 11:17:31]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-13 11:18:01]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/120001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/120001

[hiroshige, 2015-10-13 12:53:08]

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

[hiroshige, 2015-10-13 12:53:09]

yoav@,
Thanks for comments.
I added tests (and CORE_EXPORT's needed for the tests).
Could you take a look?

[commit-bot: I haz the power, 2015-10-13 12:58:43]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-13 12:58:44]

Dry run: This issue passed the CQ dry run.

[Yoav Weiss, 2015-10-14 11:36:50]

LGTM

[hiroshige, 2015-10-15 08:33:12]

The CQ bit was checked by hiroshige@chromium.org

[commit-bot: I haz the power, 2015-10-15 08:33:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/120001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/120001

[commit-bot: I haz the power, 2015-10-15 10:28:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-15 10:29:00]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[hiroshige, 2015-10-18 13:47:17]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-18 13:47:30]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/140001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/140001

[commit-bot: I haz the power, 2015-10-18 14:54:37]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-18 14:54:38]

Dry run: This issue passed the CQ dry run.

[hiroshige, 2015-10-18 15:04:27]

Fixed style ("Attribute *attributeB" to "Attribute* attributeB" etc.) in Patch
Set 5.
[pointers-cpp]
https://www.chromium.org/blink/coding-style#TOC-Pointers-and-References

[hiroshige, 2015-10-18 15:04:59]

The CQ bit was checked by hiroshige@chromium.org

[hiroshige, 2015-10-18 15:05:00]

The patchset sent to the CQ was uploaded after l-g-t-m from yoav@yoav.ws
Link to the patchset: https://codereview.chromium.org/1392863002/#ps140001
(title: "Style fix.")

[commit-bot: I haz the power, 2015-10-18 15:05:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1392863002/140001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1392863002/140001

[commit-bot: I haz the power, 2015-10-18 15:08:34]

Committed patchset #5 (id:140001)

[commit-bot: I haz the power, 2015-10-18 15:09:22]

Patchset 5 (id:??) landed as
https://crrev.com/37d6b63597f41322f6092405ae913b19a7e85686
Cr-Commit-Position: refs/heads/master@{#354704}