Make device enumeration in VideoCaptureDeviceFactory asynchronous.

Previously VideoCaptureDeviceFactory interface provided
GetDeviceDescriptors() and GetSupportedFormats(). Both were synchronous.
That hasn't worked well on some platforms, particularly on Fuchsia.
Replaced both methods with asynchronous GetDevicesInfo(). The new method
returns device descriptors as well as list formats supported by each
device.

Bug: 1072932
Change-Id: If76abbc66db9fee54140a5e352eb0f8fd39f3294
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306918


Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791485}