clang: Blow away the plugin build directory before every run.
clang: Blow away the plugin build directory before every run.

The mac trybot had and old copy of the plugin source in its build
directory as a left-over from an old version of the update script.
This caused clang's build system to build this old source instead
of the current source.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6579011

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@75795 0039d316-1c4b-4281-b951-d872f2087c98

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