This is a reland of 1b3b808a

Original change's description:
> inspector: find magic comment using V8 scanner
>
> Inspector tries to provide sourceURL and sourceMappingURL for scripts
> with parser errors. Without this CL we convert source of each script
> to inspector string and search for magic comment there. Some web sites
> use pattern when they get some data from network and constantly try to
> parse this data as JSON, in this case we do a lot of useless work.
>
> So we can parse magic comments on V8 side only for compilation errors
> (excluding parse JSON errors), to do it we can reuse scanner by running
> it on each potential comment.
>
> R=alph@chromium.org,verwaest@chromium.org,yangguo@chromium.org
>
> Bug: chromium:873865,v8:7731
> Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
> Change-Id: I77c270fd0e95cd7b2c9ee4b7f72ef344bc1fa104
> Reviewed-on: https://chromium-review.googlesource.com/1182446
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Alexei Filippov <alph@chromium.org>
> Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55280}

TBR=alph@chromium.org,verwaest@chromium.org,yangguo@chromium.org

Bug: chromium:873865, v8:7731
Change-Id: I097678fda0ebdcbd35a85be0bb6cf0fcb052bcbd
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/1195533
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55507}