Make browser_tests shardable.
Make browser_tests shardable.

Add methods to browser_tests which allow sharding
control to be used to select the tests that are
called by RunTest().  The same basic process is
used by GTest to decide if a given test should be
run (assuming the suite should be sharded).

Reuse GTEST_TOTAL_SHARDS and GTEST_SHARD_INDEX since
browser_tests is a wrapper around GTest and using the
same variable names will simplify our tools.

BUG=69423
TEST=browser_tests runs fine unsharded, can be run
sharded

Review URL: http://codereview.chromium.org/6333004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@71530 0039d316-1c4b-4281-b951-d872f2087c98

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