[prpc] pass router.Context
[prpc] pass router.Context

instead of passing pieces of router.Context, pass router.Context itself.

Bug:
Change-Id: Ide0f9dfc85e91170d0e7d7514ad960e8d436ee4a
Reviewed-on: https://chromium-review.googlesource.com/832862
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go