chrome_stages: upload partial metadata in SyncChrome
chrome_stages: upload partial metadata in SyncChrome

Uploading partial metadata that includes the chrome version in this
stage and in its HandleSkip method ensures that:
a) The chrome version is written to metadata for every build on which it
is determined.
b) The partial metadata file containing this version is uploaded as soon
as possible.

BUG=chromium:372498
TEST=lumpy-full, lumpy-paladin, master-paladin tryjobs

Change-Id: Id34e671cf83bc9ba8ff2d7f82fc09a8fd07e6579
Reviewed-on: https://chromium-review.googlesource.com/199576
Tested-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Reviewed-by: Christine Lee <06e32eabe3d11750cc47af48aa4f7367a7b04000@chromium.org>
Commit-Queue: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite