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.

BUG=467808, 526708

Review URL: https://codereview.chromium.org/1329033002

Cr-Commit-Position: refs/heads/master@{#347847}
7 files changed