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/1182446Reviewed-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}