Add ChromeSDKMock PartialMock.
Add ChromeSDKMock PartialMock.

Adds re-usable partial mock object for ChromeSDK.

BUG=None.
TEST=Unit tests.

Change-Id: I3cc61b5dcb813615b6ec154c078cb5cd022b6ffa
Reviewed-on: https://gerrit.chromium.org/gerrit/43150
Commit-Queue: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,zhang0137/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite