Add feature flag and scaffolding for usage of the
Windows.Devices.Sensors WinRT API in Chromium

As the first step for implementing usage of the
Windows.Devices.Sensors WinRT API in Chromium, this CL:
  - Introduces a feature flag (disabled by default) to switch
    between the already existing ISensor API and the new
    Windows.Devices.Sensors API implementation.
  - Adds a skeleton PlatformSensorProviderWinrt.

Verified by:
  1. Built Chromium and installed the private build.
  2. Verified ambient light sensor could be instantiated when
     the flag was in default state.
  3. Verified ambient light sensor could not be instantiated
     when flag was set to Enabled.
The new unit test was run as well.

Bug: 958266
Change-Id: I3be7fa46c7d00d87d40fc0ea9074c88a42122512
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1685853
Commit-Queue: Wenjie Shi <wensh@microsoft.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676455}
15 files changed