Merge 101979 - promo_resource_service fixes/cleanup for promos.

Merge 101979 - promo_resource_service fixes/cleanup for promos.

Remove is_synced check in CanShowPromo.
Add support for additional question param max_group_number.
Fix a bug where group was reset everytime the promo resource was fetched.
Breakup UnpackPromoSignal to a number of smaller helper functions
    for clarity, and so they can be individually unit-tested.
Put all the logic of whether to display a promo into CanShowPromo.
Get rid of unnecessary namespace PromoResourceServiceUtil.
Make IsBuildTargeted private.
Make GetChannel a private function in anonymous scope 
    to consolidate duplicated comments/code.
Delete unused web_resource_cache_.

BUG=93201
TEST=Only relevant change is removal of is_synced check in CanShowPromo.
Review URL: http://codereview.chromium.org/7655008

TBR=achuith@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=103210