This is a reland of e987606a

Original change's description:
> inspector: do not convert and store String16 for script source
>
> We need script source for:
> - calculating hash to report as part of scriptParsed event,
> - reporting it as response on getScriptSource request,
> - searching inside as response on searchInContent request,
> - breakpoints hints.
>
> In all cases there is no need to store source on inspector side.
>
> R=alph@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: Ice24ddc72cfff36fb9a2dff2d7c4543defe3f668
> Reviewed-on: https://chromium-review.googlesource.com/1182603
> Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Reviewed-by: Alexei Filippov <alph@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55286}

TBR=alph@chromium.org

Bug: chromium:873865, v8:7731
Change-Id: I0a0e93cd9d7797e4b3c57b1ab4f1a20af27fea9c
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/1195907Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55509}