This reverts commit 22cb8d45.

Reason for revert: it is fundamentally wrong to fetch default
frame context using contextGroupId: contextGroupId is per page rather
then per frame.

Original change's description:
> inspector: teach v8Inspector to return default context
>
> This is a follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/1173718
>
> R=​kozy, pfeldman
> TBR=pfeldman
>
> Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;luci.chromium.try:linux_chromium_rel_ng;master.tryserver.blink:linux_trusty_blink_rel
> Change-Id: I48b4ca5589505d03773477623654fa54703f0714
> Reviewed-on: https://chromium-review.googlesource.com/1175061
> Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
> Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55133}

TBR=lushnikov@chromium.org,pfeldman@chromium.org,kozyatinskiy@chromium.org
NOTRY=true

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ide4246bfe75ccc8a4fb1f0c5dbc44ae4236cac5c
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;luci.chromium.try:linux_chromium_rel_ng;master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1419082
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58906}