Move device_sensors cross-process headers out of //content

In preparation for migrating the implementation of
//device/sensors/public/interfaces down from //content/browser
into //device, this CL moves the headers shared between that implementation
and its client from //content/common/device_sensors into
//device/sensors/public/cpp.

I have left these headers in the //content namespace for now as they're
widely used by code that will also move. I'll move their namespace in
one go together with the rest of the code that's moving.

The Blink headers referenced from these ones should also move to sit next
to these ones.

BUG=612322

Review-Url: https://codereview.chromium.org/2389963003
Cr-Commit-Position: refs/heads/master@{#424160}
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index a8ee47a..7302df21 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -65,6 +65,7 @@
     "//device/geolocation/public/interfaces",
     "//device/nfc:mojo_bindings",
     "//device/power_save_blocker",
+    "//device/sensors/public/cpp",
     "//device/time_zone_monitor",
     "//device/vibration",
     "//device/wake_lock",