Use case: anonymous script with sourceMappingUrl. User can set
breakpoint in source with sourceUrl from sourceMap, we persist this
breakpoint in DevTools and on page reload breakpoint should be restored
correctly.

Debugger.setBreakpointByUrl method provides capabilities to set
provisional breakpoints and looks like best candidate for new "scriptHash"
argument.

I considered other options such as replacing scriptId with something
more persistent like "script-hash:script-with-this-hash-number" but it
looks more complicated and doesn't provide clear advantages.

One pager: http://bit.ly/2wkRHnt

R=pfeldman@chromium.org

Bug: chromium:459499
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0e2833fceffe6b04afac01d1a4522d6874b6067a
Reviewed-on: https://chromium-review.googlesource.com/683597
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48357}