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The --data-path switch, if passed, specifies what directory to use as the browsing
context's data directory. This CL checks that the directory actually exists/can be
created, and converts relative paths to absolute.

Note: This code is executed twice in content_shell (for normal and OTR contexts); in
the long run, giving the shell its own PathProvider may be a better solution.

Review-Url: https://codereview.chromium.org/2344073003
Cr-Commit-Position: refs/heads/master@{#422670}