Remove RuntimeAPIDelegate::GetPreviousVersion.

We currently go to chrome/ to get the extension version, but we don't
need to. ExtensionRegistry in extensions/ has the info already, move
the function to ExtensionRegistry.

Also, the name GetPreviousVersion is misleading. It only gives us the
current version, so rename it to be simply GetVersion.

BUG=723754
Test=None, internal only change.

Review-Url: https://codereview.chromium.org/2892843002
Cr-Commit-Position: refs/heads/master@{#473332}
10 files changed