Idle: Add ScopedSetIdleState to make ui/base/idle more test-friendly

This CL adds a new ScopedSetIdleState class for testing that sets an
overriding idle state in ui/base/idle so that classes depending on
ui/base/idle can be more easily tested.

(cherry picked from commit ecd097eeadf2ba1df20c0c269cedfc80912ebeb7)

Bug: 951560
Change-Id: I4234af0783c7fc053620803b74f0ef6b0fc0ce4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643414
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#666444}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653869
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/branch-heads/3809@{#234}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

• ui/base/idle/BUILD.gn[diff]
• ui/base/idle/idle.cc[diff]
• ui/base/idle/idle_android.cc[diff]
• ui/base/idle/idle_chromeos.cc[diff]
• ui/base/idle/idle_internal.cc[Added - diff]
• ui/base/idle/idle_internal.h[Added - diff]
• ui/base/idle/idle_linux.cc[diff]
• ui/base/idle/idle_mac.mm[diff]
• ui/base/idle/idle_win.cc[diff]
• ui/base/idle/scoped_set_idle_state.cc[Added - diff]
• ui/base/idle/scoped_set_idle_state.h[Added - diff]