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: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@75795 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser