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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 find the list of xctest files under each of
the application targets and then associate them with the corresponding xctest
target.

This CL associates xctest files with xctest targets and because our compilation
is done via ninja, it also adds '-help' as per file compiler flags to each
xctest file reference to prevent Xcode from trying to compile the xctest files.

BUG=614818

Review-Url: https://codereview.chromium.org/2616773003
Cr-Commit-Position: refs/heads/master@{#442120}