Build color_unittests only with flag enabled

Use 'use_color_pipeline' build flag to gate the build and run of
color_unittests.

Since the flag is false by default, remove the unittest from buildbots
for now.

BUG=1003612

Change-Id: Idff6eefb720269e15c60ca69335a5eb0533bd592
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947225
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721264}

• BUILD.gn[diff]
• testing/buildbot/chrome.json[diff]
• testing/buildbot/chromium.chromiumos.json[diff]
• testing/buildbot/chromium.clang.json[diff]
• testing/buildbot/chromium.fyi.json[diff]
• testing/buildbot/chromium.linux.json[diff]
• testing/buildbot/chromium.mac.json[diff]
• testing/buildbot/chromium.memory.json[diff]
• testing/buildbot/chromium.win.json[diff]
• testing/buildbot/gn_isolate_map.pyl[diff]
• testing/buildbot/test_suites.pyl[diff]
• tools/code_coverage/test_suite.txt[diff]