Revert "Initial work toward making ///mojo/public flexible wrt root dir."

This reverts commit b419edd2bc8181c59f60f2621a31b592c83d32b1.

Speculative revert in order to see if it fixes the compile failures on the
bots, which don't seem obviously related but perhaps are in some subtle way.

TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/789483002