[mojo] Add testing support to BrowserInterfaceBrokerProxy

This change adds a SetBinderForTesting() method to
BrowserInterfaceBrokerProxy and modifies MojoInterfaceInterceptor to
call this new method if a script passes a new "useBrowserInterfaceBroker"
parameter.
Also converts IdleManager to use BrowserInterfaceBrokerProxy as a use
case for overriding interface handlers in web tests.

Bug: 985118
Change-Id: I8dc3fc1c4beb2ae375a2534b03cd890ce1e3b52c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1756480
Commit-Queue: Oksana Zhuravlova <[email protected]>
Reviewed-by: Ken Rockot <[email protected]>
Reviewed-by: Tom Sepez <[email protected]>
Reviewed-by: Hiroki Nakagawa <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Robert Ma <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Ayu Ishii <[email protected]>
Cr-Commit-Position: refs/heads/master@{#688743}
26 files changed