Introduce explicit constant for per Context debug data set by embedder

Introduce explicit constant for per Context debug data set by embedder

It's value is hardwired in gin[1] and is referenced in Blink[2]. Since it is treated specially by v8 debugger it should be defined in v8 API and referenced in gin and blink..

[1] https://src.chromium.org/viewvc/chrome/trunk/src/gin/public/context_holder.h?r1=239099&r2=239098&pathrev=239099
[2] https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/core/v8/V8PerContextData.cpp&q=kDebugIdIndex%20f:V8PerContextData&sq=package:chromium&type=cs

BUG=chromium:466631
LOG=Y

Committed: https://crrev.com/c622cb637a2831f9c2bf0ec7b80bb65440e9afc4
Cr-Commit-Position: refs/heads/master@{#27290}

[yurys, 2015-03-18 10:19:40]

yurys@chromium.org changed reviewers:
+ jochen@chromium.org

[yurys, 2015-03-18 10:20:09]

yurys@chromium.org changed reviewers:
+ yangguo@chromium.org

[Yang, 2015-03-18 12:31:04]

On 2015/03/18 10:20:09, yurys wrote:

LGTM.

[yurys, 2015-03-18 12:51:33]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2015-03-18 12:51:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1020453002/1

[commit-bot: I haz the power, 2015-03-18 13:03:09]

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

[commit-bot: I haz the power, 2015-03-18 13:03:10]

Try jobs failed on following builders:
  v8_presubmit on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_presubmit/builds/1330)

[Yang, 2015-03-18 13:05:55]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2015-03-18 13:06:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1020453002/1

[commit-bot: I haz the power, 2015-03-18 13:08:31]

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

[commit-bot: I haz the power, 2015-03-18 13:08:32]

Try jobs failed on following builders:
  v8_presubmit on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_presubmit/builds/1333)

[yurys, 2015-03-19 08:17:11]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2015-03-19 08:17:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1020453002/1

[commit-bot: I haz the power, 2015-03-19 08:18:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-19 08:18:54]

Patchset 1 (id:??) landed as
https://crrev.com/c622cb637a2831f9c2bf0ec7b80bb65440e9afc4
Cr-Commit-Position: refs/heads/master@{#27290}