Update XRRigidTransform Constructor checks

This addresses feedback submitted to crrev.com/c/1725319 after it was
submitted.  Mainly around using exact equality for whole numbers and
the math around a length of zero.

Bug: 977703
Change-Id: Ia43f204a9f754077cda6bd74b2e901f724c7ee9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1727571
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682538}

third_party/blink/renderer/modules/xr/xr_rigid_transform.cc

@@ -12,12 +12,6 @@
 #include "third_party/blink/renderer/platform/bindings/exception_state.h"
 #include "third_party/blink/renderer/platform/transforms/transformation_matrix.h"
 
-namespace {
-bool IsWithinEpsilon(double a, double b) {
-  return std::abs(a - b) < std::numeric_limits<double>::epsilon();
-}
-}  // namespace
-
 namespace blink {
 
 // makes a deep copy of transformationMatrix
@@ -68,16 +62,21 @@ XRRigidTransform::XRRigidTransform(DOMPointInit* position,
 XRRigidTransform* XRRigidTransform::Create(DOMPointInit* position,
                                            DOMPointInit* orientation,
                                            ExceptionState& exception_state) {
-  if (position && !IsWithinEpsilon(1.0, position->w())) {
+  if (position && position->w() != 1.0) {
     exception_state.ThrowTypeError("W component of position must be 1.0");
     return nullptr;
   }
 
   if (orientation) {
-    if (IsWithinEpsilon(orientation->x(), 0.0) &&
-        IsWithinEpsilon(orientation->y(), 0.0) &&
-        IsWithinEpsilon(orientation->z(), 0.0) &&
-        IsWithinEpsilon(orientation->w(), 0.0)) {
+    double x = orientation->x();
+    double y = orientation->y();
+    double z = orientation->z();
+    double w = orientation->w();
+    double sq_len = x * x + y * y + z * z + w * w;
+
+    // The only way for the result of a square root to be 0 is if the squared
+    // number is 0, so save the square root operation and just compare to 0 now.
+    if (sq_len == 0.0) {
       exception_state.ThrowDOMException(DOMExceptionCode::kInvalidStateError,
                                         "Orientation's length cannot be 0");
       return nullptr;