Disable extension Panels on Dev and Canary, except for the whitelist
They're already disabled on Beta and Stable channels. Leaving them
enabled in Dev/Canary confuses users and makes people suspect
After this, panels are only enabled when --enable-panels is set in
chrome://flags and for the whitelist (Hangouts).
CL also tries to consolidate all the Hangouts extension IDs scattered
around the codebase.