Unit test need not provide a Default.png file.

Unit test need not provide a Default.png file.

testing/gtest.gyp already provides a Default.png in "mac_bundle_resources"
so tests dependening on that target need not provide such a file otherwise
we get a warning from ninja due to multiple rules generating the same
output.

Fix the following warning:
ninja: warning: multiple rules generate crnet_test.app/Default.png. builds involving this target will not be correct; continuing anyway [-w dupbuild=warn]

TEST=Delete out/Debug/crnet_test.app, rebuild it with the patch applied and
check that 1. the warning is no longer displayed, 2. that a file named
out/Debug/crnet_test.app/Default.png exists.

BUG=520560

Review URL: https://codereview.chromium.org/1294753002

Cr-Commit-Position: refs/heads/master@{#343640}