I can do a simple solution to this where the user gives a custom chromium commit hash 
when they start the server. 

The issue with a more automagically solution that relies on the particular checkout 
of the front-end version is that we would need to inspect their current git commit and, 
if they're using the devtools-frontend github repo, we would need to scrape the chromium commit
from the commit description. Doing this would require significant interfacing with git and 
perhaps using an npm module like nodegit. 

I would suggest sticking with the simple solution.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/846251ff8e503e1a90ce93a5119d32010e8c872f

commit 846251ff8e503e1a90ce93a5119d32010e8c872f
Author: chenwilliam <chenwilliam@chromium.org>
Date: Fri Aug 12 18:47:12 2016

DevTools: Override browser commit hash with process.env.CHROMIUM_COMMIT

This is a simple solution for hosted mode users who want to override which
version of the proxied files (SupportedCSSProperties.js and InspectorBackendCommands.js)
they want to use, instead of relying on the default which gets the commit hash
of the browser/back-end.

Example:
CHROMIUM_COMMIT=813208daf9e9821657e55ef7c40a14815efdcf49 npm run server

Will update docs after this lands.

Added a comment to the bug issue for more context on why we used this approach.

BUG= 635136 

Review-Url: https://codereview.chromium.org/2236733002
Cr-Commit-Position: refs/heads/master@{#411721}

[modify] https://crrev.com/846251ff8e503e1a90ce93a5119d32010e8c872f/third_party/WebKit/Source/devtools/scripts/hosted_mode/server.js