[filesapp] Add a browser_test for DevToolsListener class

Test that JS code coverage of a JS IFFE injected in a WebContents gets
collected by DevToolsListener and stored in a ScopedTempDir.

Test: browser_tests --gtest_filter="*DevToolsListener*"
Bug: 1113941
Change-Id: I8f04f2e704d677823ffa154272605798b7046f77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461602
Commit-Queue: Ben Reich <benreich@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816064}