Mark Generic Sensors API "stable"

The Generic Sensors API has already been shipped however in
runtime_enabled_features.json5 it was still set to "experimental"
because it was shipped by setting the default value of the
device::kGenericSensors flag to enabled. This change reflects that in
Blink and reverses the logic in runtime_features.cc so that disabling
the Chrome flag will disable the Blink flag.

Bug: 750018
Change-Id: If11794bc915f6e73e82c55efcda50c5f58859a0a
Reviewed-on: https://chromium-review.googlesource.com/1128396
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573342}
11 files changed