Create package keywords directory if it doesn't exist.

bin/cbuildbot.py

Index: bin/cbuildbot.py
diff --git a/bin/cbuildbot.py b/bin/cbuildbot.py
index 90e965bb4f496c18aa4dd3f1d3cddec851bfe62a..08a0dcf065353f8ed835a3f085b7cddd187e0e8b 100755
--- a/bin/cbuildbot.py
+++ b/bin/cbuildbot.py
@@ -232,10 +232,12 @@ def _MarkChromeAsStable(buildroot, tracking_branch, chrome_rev, board):
     return None
   else:
     chrome_atom = portage_atom_string.split('=')[1]
+    keywords_file = CHROME_KEYWORDS_FILE % {'board': board}
     # TODO(sosa): Workaround to build unstable chrome ebuild we uprevved.
-    RunCommand(['sudo', 'tee', CHROME_KEYWORDS_FILE % {'board': board}],
-               input='=%s\n' % chrome_atom, enter_chroot=True,
-               cwd=cwd)
+    RunCommand(['sudo', 'mkdir', '-p', os.path.dirname(keywords_file)],
+               enter_chroot=True, cwd=cwd)
+    RunCommand(['sudo', 'tee', keywords_file], input='=%s\n' % chrome_atom,
+               enter_chroot=True, cwd=cwd)
     return chrome_atom