[devtools] Run resumption and get-script-source through IO thread

Since instrumentation breakpoints will pause message processing on the
main thread, we need to be able to resume the pause on the IO thread.

Additionally, we need to be able to get script source to place
breakpoints while message processing is suspended.

Bug: chromium:1354043
Change-Id: Idd1d3cb31abad0061ba47ef78a3a2c8d859d4ca9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3949315
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1058521}