This piggy-backs on top of existing precise and best-effort coverage to expose
block coverage through the inspector protocol.

Coverage collection now implicitly reports block-granularity coverage when
available.  A new 'isBlockCoverage' property on Inspector's FunctionCoverage
type specifies the granularity of reported coverage.

For now, only count-based block coverage is supported, but binary block
coverage should follow soon.

Support is still gated behind the --block-coverage flag.

Bug: v8:6000
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I9c4d64e1d2a098e66178b3a68dcee800de0081af
Reviewed-on: https://chromium-review.googlesource.com/532975
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46157}