Only checkout chromite when target_os=chromeos.

I'm having a tough time finding context behind the "pyelftools"
dependency. It might no longer be relevant, in which case we could
restrict chromite to only CrOS checkouts. Worth a shot.

Bug: 937821
Change-Id: I96ae00a272390042b466ae50defb18e9bf3eafc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323823
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#792746}

DEPS

@@ -873,10 +873,10 @@ deps = {
       'dep_type': 'cipd',
   },
 
-  # Build tools for Chrome OS. Note: This depends on third_party/pyelftools.
+  # Build tools for Chrome OS.
   'src/third_party/chromite': {
       'url': Var('chromium_git') + '/chromiumos/chromite.git' + '@' + '1da974976b2a88fb094bc5c87dcf775db5a1ac68',
-      'condition': 'checkout_linux',
+      'condition': 'checkout_chromeos',
   },
 
   'src/third_party/cld_3/src':
@@ -1277,7 +1277,7 @@ deps = {
       'dep_type': 'cipd',
   },
 
-  # Dependency of chromite.git and skia.
+  # Dependency of skia.
   'src/third_party/pyelftools': {
       'url': Var('chromium_git') + '/chromiumos/third_party/pyelftools.git' + '@' + '19b3e610c86fcadb837d252c794cb5e8008826ae',
       'condition': 'checkout_linux',