Example case:
After the change in https://codereview.chromium.org/2642193011, the baseline for paint/invalidation/control-clip.html needed to be updated but the original baseline should have been kept for virtual/stable/paint/invalidation/control-clip.html.

The "copy-existing-baselines" stage (https://cs.chromium.org/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline.py?l=136) normally copies existing baselines for a particular platform X over to any baseline directories that would use the baseline directory for X as a fall-back.

Proposed change:
Suppose we're going to download a new baseline for mac10.11 for test x/y/z.html which has a virtual test in the virtual test suite "foo". The old baseline should be copied over to both platform/mac-10.10/x/y/z-expected.txt as well as virtual/foo/platform/mac/x/y/z-expected.txt

Then, in the "optimize-baselines" stage, all of the platform-specific baselines in platform/*/virtual/foo/x/y/z-expected.txt could be de-duplicated, perhaps leaving behind a baseline in virtual/foo/x/y/z-expected.txt, or platform/mac/virtual/foo/x/y/z-expected.txt.