Fix controller positioning in VrShell when gyro reads are small

BUG=716201

Review-Url: https://codereview.chromium.org/2853843002
Cr-Commit-Position: refs/heads/master@{#468375}

chrome/browser/android/vr_shell/elbow_model.cc

@@ -32,9 +32,7 @@ constexpr float kDefaultElbowRotationRatio = 0.4f;
 }  // namespace
 
 ElbowModel::ElbowModel(gvr::ControllerHandedness handedness)
-    : handedness_(handedness),
-      alpha_value_(1.0f),
-      torso_direction_{0.0f, 0.0f, 0.0f} {}
+    : handedness_(handedness), alpha_value_(1.0f), torso_direction_(kForward) {}
 
 ElbowModel::~ElbowModel() = default;
 

device/vr/vr_math.cc

@@ -120,6 +120,9 @@ float NormalizeVector(gfx::Vector3dF* vec) {
 void NormalizeQuat(Quatf* quat) {
   float len = sqrt(quat->qx * quat->qx + quat->qy * quat->qy +
                    quat->qz * quat->qz + quat->qw * quat->qw);
+  DCHECK_NE(len, 0);
+  if (len == 0)
+    return;
   quat->qx /= len;
   quat->qy /= len;
   quat->qz /= len;