Prepare IssuesModel for 1:n mapping between backend and frontend issues
Prepare IssuesModel for 1:n mapping between backend and frontend issues

Each received backend issue can result in multiple issues in the
frontend. This CL implements a simple mechanism where we can provide
(optional) functions per issue code that know how to handle a backend
issue.

R=f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org

Change-Id: I37ca1f50a745fc8cfb22444d79dfa3fed469aa25
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2123468
Reviewed-by: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>

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