Don't overwrite existing prebuilts when Chrome is pinned.
Don't overwrite existing prebuilts when Chrome is pinned.

Whenever Chrome is pinned to an old version, the Chrome PFQ should stop
updating prebuilts, and just leave Chrome the way it is. This ensures
that developers continue to have fast builds even when Chrome is pinned.

To implement this, I've updated the Chrome PFQ to not unpin Chrome anymore.
This unpinning is only actually needed for builders that build versions of
Chrome other than the latest.

By itself, the above change would make the Chrome PFQs build and upload
prebuilts for the current version of Chrome and not actually uprev Chrome.
That would be a good thing, but it would be confusing because the Chrome
PFQ would be 'green' and therefore developers would think that Chrome is
being uprevved. To make things more clear, I've also added a sanity check
to the uprev stage to make sure that the new version of Chrome can be emerged.
If Chrome is pinned to an old version, we will not be able to emerge the new
version of Chrome because emerge will return an warning explaining that Chrome
has likely been pinned to an old version.

BUG=chromium-os:31081
TEST=Trybot runs. Chrome PFQ should leave Chrome alone if Chrome is pinned,
     but informational builders should still build new versions of Chrome.

Change-Id: I3b71b9b1139b14f1a0a211750bee71deb6e9e4fc
Reviewed-on: https://gerrit.chromium.org/gerrit/27458
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Ready: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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