Add support for --target flag to webrtc_tests.py.
Add support for --target flag to webrtc_tests.py.

In https://codereview.chromium.org/26190002 Chromium
started a cleanup of their wrapper script, by adding
the --target flag and start passing build dir similar
to how the bots are setting it (i.e. pass out and not
out/Release).
This CL adds --target support for our wrapper script,
without changing the existing behavior (I'll do a
larger update at a later stage).

BUG=none
TEST=Ran the following successfully:
tools/valgrind-webrtc/webrtc_tests.sh --build_dir out/Release --target Release --test test_support_unittests
tools/valgrind-webrtc/webrtc_tests.sh --build_dir out --target Release --test test_support_unittests
tools/valgrind-webrtc/webrtc_tests.sh --target Release --test  out/Release/test_support_unittests
TBR=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2396004

git-svn-id: 917f5d3ca488f358c4d40eaec14422cf392ccec9@4957 4adac7df-926f-26a2-2b94-8c16560cd09d

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