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This CL is a step toward deep linking to site details in the site
settings. This CL adds the c++ handler backend and does show the
exceptions in the UI. The page title is not updated with the subject URL,
but that is not a regression and will be addressed in a future CL.

BUG=635874
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2338163004
Cr-Commit-Position: refs/heads/master@{#419071}