Fix RunTests to not stall on empty test list
Fix RunTests to not stall on empty test list

Some PRESUBMIT files dynamically generate test list
and so may pass an empty list to RunTests.  There
appears to be a bug in multiprocessing.Pool that
causes a stall in map_async with an empty list.

TBR=maruel

Review URL: https://chromiumcodereview.appspot.com/14962011

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@200613 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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