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Move the target_defaults into its own gypi file so that we can
parse the variables in khronos_glcts.gypi with the gypi_to_gn.py
script.  The gypi_to_gn.py chokes on target_defaults.

BUG=chromium:412865, chromium:471903
R=kbr@chromium.org, piman@chromium.org, dpranke@chromium.org
TEST=Verify khronos_glcts_test builds and runs
Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>

Review URL: https://codereview.chromium.org/1427433007

Cr-Commit-Position: refs/heads/master@{#356711}