Halve the number of lookups in ExtractLocationForJSFunction() by calling
GetPositionInfo() directly instead of making separate calls for column
and line number.

Improve the efficiency of position lookups in slow mode. The current
code does a linear walk through the source by calling String::Get() for
each character. This PR also does a linear walk, but avoids the overhead
of multiple Get() calls by pulling the String's flat content into a
local vector and walking through that.

Downstream Electron discussion of this can be found at
https://github.com/electron/electron/issues/24509

Apologies in advance if I've missed anything; this is my first V8 CL...

Change-Id: I22b034dc1bfe967164d2f8515a9a0c1d7f043c83
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2496065
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70783}