sensors: Read Euler angles instead of quaternions from WebDriver

As a result of w3c/deviceorientation#124 and
w3c/orientation-sensor#83.

The reading format is alpha-beta-gamma as described in the spec. They
are measured in degrees and must fall within certain ranges. Internally,
however, we always perform the Euler angles to quaternions conversion at
the edges (i.e. in ChromeDriver and the Internals implementation used by
content_shell), so that the the CDP and //services layers remain
unchanged and continue to support only quaternions and the
{ABSOLUTE,RELATIVE}_ORIENTATION_QUATERNION types for simplicity.

The code to convert Euler angles to quaternions was copied from
SensorInspectorAgent in Blink and is available for use by any callers
that need to validate Euler angles and convert them. The original code
remains in place because the entirety of the SensorInspectorAgent will
be removed soon due to the work on bug 1506995.

The test values for the orientation-sensor web tests had to be adapted:
we now provide the inputs as Euler angles. The expected values have
changed slightly as we had to find Euler _and_ quaternion values that
were easy enough to read.

Written in collaboration with Juha Vainio <juha.j.vainio@intel.com>

Bug: 1506995, 1520912, 1520919
Change-Id: I047f41f172f0bbcf30c7462926cec7ae0a66d4e5

orientation-sensor/resources/sensor-data.js

@@ -15,9 +15,13 @@ const kRelativeOrientationSensorData = {
 };
 
 const kOrientationReadings = {
-  readings: [{quaternion: [1, 0, 0, 0]}],
-  expectedReadings: [{quaternion: [1, 0, 0, 0]}],
-  expectedRemappedReadings: [{quaternion: [-0.70710678, 0.70710678, 0, 0]}]
+  // WebDriver input data must be given in Euler angles according to
+  // https://w3c.github.io/deviceorientation/#parse-orientation-data-reading-algorithm
+  // and converted to quaternions via
+  // https://w3c.github.io/orientation-sensor/#create-a-quaternion-from-euler-angles.
+  readings: [{alpha: 0, beta: -180, gamma: 0}],
+  expectedReadings: [{quaternion: [-1, 0, 0, 0]}],
+  expectedRemappedReadings: [{quaternion: [0.70710678, -0.70710678, 0, 0]}]
 };
 
 const kRotationMatrix = [1,  0,  0, 0,