If there had been no debug listener v8::Debug::GetDebugContext would have created new context and wouln't have kept reference to it. This way we may well end up with several debug contexts and disabled debugger.

As a side effect this change allows to efficiently distinguish debug context from blink contexts by simply comparing handles.

BUG=chromium:482290
LOG=Y

Review URL: https://codereview.chromium.org/1136733002

Cr-Commit-Position: refs/heads/master@{#28356}

Revert of Revert of Debugger: deduplicate shared function info when setting script break points. (patchset #1 id:1 of https://codereview.chromium.org/999273003/)

Reason for revert:
Reland since the failure has been fixed in https://codereview.chromium.org/1035523005/

Original issue's description:
> Revert of Debugger: deduplicate shared function info when setting script break points. (patchset #4 id:60001 of https://codereview.chromium.org/998253005/)
>
> Reason for revert:
> Code caching failures.
>
> Original issue's description:
> > Debugger: deduplicate shared function info when setting script break points.
> >
> > Also fix Debug.showBreakPoints for multiple break points at the same location.
> >
> > BUG=v8:3960
> > LOG=N
> >
> > Committed: https://crrev.com/73b17a71a22564c0b66d9aa7c00948c748f5b290
> > Cr-Commit-Position: refs/heads/master@{#27444}
>
> TBR=mstarzinger@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=v8:3960
>
> Committed: https://crrev.com/9b29d008dfcc00bf56be8040add1d2c5e404673b
> Cr-Commit-Position: refs/heads/master@{#27448}

TBR=mstarzinger@chromium.org
BUG=v8:3960
LOG=N

Review URL: https://codereview.chromium.org/1037013002

Cr-Commit-Position: refs/heads/master@{#27472}

Revert of Debugger: deduplicate shared function info when setting script break points. (patchset #4 id:60001 of https://codereview.chromium.org/998253005/)

Reason for revert:
Code caching failures.

Original issue's description:
> Debugger: deduplicate shared function info when setting script break points.
>
> Also fix Debug.showBreakPoints for multiple break points at the same location.
>
> BUG=v8:3960
> LOG=N
>
> Committed: https://crrev.com/73b17a71a22564c0b66d9aa7c00948c748f5b290
> Cr-Commit-Position: refs/heads/master@{#27444}

TBR=mstarzinger@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:3960

Review URL: https://codereview.chromium.org/999273003

Cr-Commit-Position: refs/heads/master@{#27448}

Also fix Debug.showBreakPoints for multiple break points at the same location.

BUG=v8:3960
LOG=N

Review URL: https://codereview.chromium.org/998253005

Cr-Commit-Position: refs/heads/master@{#27444}