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

Review URL: https://codereview.chromium.org/1020453002

Cr-Commit-Position: refs/heads/master@{#27290}