Continuation of:
https://codereview.chromium.org/2620343005/

This removes usage of metadata files entirely. Instead we extract
the instrumentation about source files from the test cases.

This also adds extra output of the original source file in the
detailed failure text for easier debugging. The hashes alone
made it hard to reason.

BUG=chromium:673246
NOTRY=true
TBR=tandrii@chromium.org,mbarbella@chromium.org

Review-Url: https://codereview.chromium.org/2634743004
Cr-Commit-Position: refs/heads/master@{#42371}