A high level design for this feature is as following:

1. Add an required argument called, top_level_dir.
2. By running "llvm-cov report" and parse the report, we can obtain a list of files under top_level_dir that was exercised as well as their coverage stats.
3. Using "llvm-cov show" to generate line-by-line code coverage report for the files mentioned in step 2.
4. Assumble directory based code coverage report for the files mentioned in step 2.

I'll be careful with the implementation so that it can be easily adapted to support generating coverage breakdown by components.
Please let me know if you have any questions or concerns.

Unfortunately, this is still blocked on the Clang roll, there is a CL in flight: https://chromium-review.googlesource.com/c/chromium/src/+/845700, hopefully, it will land soon.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5

commit ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5
Author: Yuke Liao <liaoyuke@chromium.org>
Date: Fri Jan 05 19:46:51 2018

[Coverage] Generate per directory code coverage report.

This CL generates per directory code coverage breakdown.

Bug:  789692 
Change-Id: I318e2b03c255069b9ecf6d10ca276264491b7d76
Reviewed-on: https://chromium-review.googlesource.com/814598
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527350}
[modify] https://crrev.com/ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5/tools/code_coverage/coverage.py
[add] https://crrev.com/ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5/tools/code_coverage/directory_example_report.html
[add] https://crrev.com/ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5/tools/code_coverage/html_templates/footer.html
[add] https://crrev.com/ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5/tools/code_coverage/html_templates/header.html
[add] https://crrev.com/ea228d0c66005f8deafcb7d1e1ebeca7fe4209c5/tools/code_coverage/html_templates/table.html