Reland of DevTools: extract V8DebuggerAgent interface, move it under .../inspector/v8

Reland of DevTools: extract V8DebuggerAgent interface, move it under .../inspector/v8 (patchset #1 id:1 of https://codereview.chromium.org/1370373004/ )

Reason for revert:
The patch did not break main bots and should not have been reverted.

Original issue's description:
> Revert of DevTools: extract V8DebuggerAgent interface, move it under .../inspector/v8 (patchset #5 id:80001 of https://codereview.chromium.org/1367623006/ )
>
> Reason for revert:
> Broke oilpan:
>
> In file included from gen/blink/core/InspectorInstrumentationImpl.cpp:10:
> ../../third_party/WebKit/Source/core/inspector/AsyncCallTracker.h:58:5: error: only virtual member functions can be marked 'override'
>     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(AsyncCallTracker);
>     ^
> ../../third_party/WebKit/Source/platform/heap/GarbageCollected.h:210:53: note: expanded from macro 'WILL_BE_USING_GARBAGE_COLLECTED_MIXIN'
> #define WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(TYPE) USING_GARBAGE_COLLECTED_MIXIN(TYPE)
>                                                     ^
> ../../third_party/WebKit/Source/platform/heap/GarbageCollected.h:199:5: note: expanded from macro 'USING_GARBAGE_COLLECTED_MIXIN'
>     DEFINE_GARBAGE_COLLECTED_MIXIN_METHODS(blink::Visitor*, TYPE)       \
>     ^
> ../../third_party/WebKit/Source/platform/heap/GarbageCollected.h:134:47: note: expanded from macro 'DEFINE_GARBAGE_COLLECTED_MIXIN_METHODS'
>     void adjustAndMark(VISITOR visitor) const override
>
> Original issue's description:
> > DevTools: extract V8DebuggerAgent interface, move it under .../inspector/v8
> >
> > V8DebuggerAgent was split into interface and implementation. All other agents should depend on the public part only.
> >
> > Once InjectedScript, InjectedScriptHost and InjectedScriptManager are also moved to .../inspector/v8 some of the methods on V8DebuggerAgent will be removed and called only by the implementation classes.
> >
> > BUG=439376
> >
> > Committed: https://crrev.com/80862359ba867d2807e9ba7b366b352ee5cfda2b
> > Cr-Commit-Position: refs/heads/master@{#351166}
>
> TBR=dgozman@chromium.org,pfeldman@chromium.org,yurys@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=439376
>
> Committed: https://crrev.com/3ef449bafc04114479f0e6a17ffdbc9fe7419b27
> Cr-Commit-Position: refs/heads/master@{#351228}

TBR=dgozman@chromium.org,pfeldman@chromium.org,haraken@chromium.org,thestig@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=439376
Committed: https://crrev.com/0a732f39b49fe5f3a2f6e15833bea85c3b5f68ab
Cr-Commit-Position: refs/heads/master@{#351269}

[yurys, 2015-09-29 08:26:20]

Created Reland of DevTools: extract V8DebuggerAgent interface, move it under
.../inspector/v8

[commit-bot: I haz the power, 2015-09-29 08:27:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1374863002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1374863002/1

[commit-bot: I haz the power, 2015-09-29 08:29:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-29 08:29:55]

Patchset 1 (id:??) landed as
https://crrev.com/0a732f39b49fe5f3a2f6e15833bea85c3b5f68ab
Cr-Commit-Position: refs/heads/master@{#351269}

[haraken, 2015-09-29 08:36:38]

LGTM, but you should have fixed the breakage since you know the CL breaks the
oilpan bot...