Add UMA for possible radio wake-up triggers on Android

This CL adds a histogram which is recorded when a URLLoader is created
and the radio activity level is dormant. The purpose of collecting the
UMA is to understand what kind of network activities trigger radio
wake-ups in the field. Previous experiments we ran suggest that radio
wake-ups could be a major source of power consumption. The UMA would
give us some insights into ways to reduce power consumption.

The UMA is recorded when the corresponding feature flag is enabled
since recording the UMA adds overheads.

Bug: 1232623
Change-Id: I45982b551745d924528fd0316dbb807f5ae8d25c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3065592


Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908698}