cros_chrome_sdk.py: Fallback to partial-metadata.json when needed.

BUG=chromium:371079
TEST=cros chrome-sdk --internal --board lumpy --version=5854.0.0 now works

Review URL: https://codereview.chromium.org/291483004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270948 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -492,7 +492,7 @@ deps_os = {
     # Build tools for targeting ChromeOS.
     "src/third_party/chromite":
       Var("chromiumos_git") + "/chromite.git" +
-      "@63193a4393f6e964bfca992bfb03e40183554f7d",
+      "@b310410e13e34af7d3f3f15be98688b016f2222f",
 
     # Dependency of chromite.git.
     "src/third_party/pyelftools":