WebVR: fix angular velocity calculation for headset

The refactor https://chromium-review.googlesource.com/536475 lost a pair of
parentheses, so the division by epsilon_seconds was just dividing one of the
matrix elements instead of the difference. This broke angular velocity
calculation.

BUG=735155

Review-Url: https://codereview.chromium.org/2947843003
Cr-Commit-Position: refs/heads/master@{#480946}

device/vr/android/gvr/gvr_delegate.cc

@@ -66,9 +66,10 @@ gfx::Vector3dF GetAngularVelocityFromPoses(gfx::Transform head_mat,
   // provided by the caller.
   for (int j = 0; j < 3; ++j) {
     for (int i = 0; i < 3; ++i) {
-      delta_mat.matrix().set(j, i,
-                             head_mat_2.matrix().get(j, i) -
-                                 head_mat.matrix().get(j, i) / epsilon_seconds);
+      delta_mat.matrix().set(
+          j, i,
+          (head_mat_2.matrix().get(j, i) - head_mat.matrix().get(j, i)) /
+              epsilon_seconds);
       inverse_head_mat.matrix().set(j, i, head_mat.matrix().get(i, j));
     }
     delta_mat.matrix().set(j, 3, 0);