[PE] Fix edge-cases in SVGGeometryElement::PathLengthScaleFactor

Update the computation of the scale factor to match what the spec [1]
has to say about a value of zero: i.e yield (near) infinity rather than
zero. Also update the case where the computed path length is zero.

[1] https://svgwg.org/svg2-draft/paths.html#PathLengthAttribute

Bug: 803127
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I6816b6bdd47cbaee90e8ad22e16d1dcf44498c7d
Reviewed-on: https://chromium-review.googlesource.com/893184Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#533003}

third_party/WebKit/Source/core/svg/SVGGeometryElement.cpp

@@ -142,15 +142,25 @@ float SVGGeometryElement::PathLengthScaleFactor() const {
   if (!pathLength()->IsSpecified())
     return 1;
   float author_path_length = pathLength()->CurrentValue()->Value();
+  // https://svgwg.org/svg2-draft/paths.html#PathLengthAttribute
+  // "A negative value is an error"
   if (author_path_length < 0)
     return 1;
-  if (!author_path_length)
-    return 0;
   DCHECK(GetLayoutObject());
+  // If the computed path length is zero, then the scale factor will
+  // always be zero except if the author path length is also zero - in
+  // which case performing the division would yield a NaN. Avoid the
+  // division in this case and always return zero.
   float computed_path_length = ComputePathLength();
   if (!computed_path_length)
-    return 1;
-  return computed_path_length / author_path_length;
+    return 0;
+  // "A value of zero is valid and must be treated as a scaling factor
+  //  of infinity. A value of zero scaled infinitely must remain zero,
+  //  while any value greater than zero must become +Infinity."
+  // However, since 0 * Infinity is not zero (but rather NaN) per
+  // IEEE, we need to make sure to clamp the result below - avoiding
+  // the actual Infinity (and using max()) instead.
+  return clampTo<float>(computed_path_length / author_path_length);
 }
 
 LayoutObject* SVGGeometryElement::CreateLayoutObject(const ComputedStyle&) {