[telemetry] find_dependencies: Skip abstract test classes when searching for page set data.


BUG=None.
TEST=tools/telemetry_tools/find_dependencies content/test/gpu/run_gpu_test --include-page-set-data  # Doesn't crash.

Review URL: https://codereview.chromium.org/174543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252463 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252463