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Previously, mojo_variables.gypi hard-coded a DEPTH of 1, which meant that only
GYP files that were directly in folders in the root could use it. After this
CL, GYP files at any depth of the tree can use it.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/561233004

Cr-Commit-Position: refs/heads/master@{#294907}