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Previously, our generated Xcode project has the issue that Xcode couldn't find
our xctests. And to fix it, one must first find the list of xctest files
under each of the application targets and then associate them with the
corresponding xctest target.

This CL enables identifying application targets and also find the list of xctest
files (xctests.mm and egtest.mm) under them recursively.

BUG=614818

Review-Url: https://codereview.chromium.org/2574643002
Cr-Commit-Position: refs/heads/master@{#441482}