Clamp argument to SVGGeometryElement.getPointAtLength to [0, length]

Per:

  https://svgwg.org/svg2-draft/types.html#__svg__SVGGeometryElement__getPointAtLength

Bug: 847198
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I9067ce9f196453bd54560d5119ebc38d35c8ac86
Reviewed-on: https://chromium-review.googlesource.com/1075332Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#562422}

third_party/WebKit/LayoutTests/external/wpt/svg/types/scripted/SVGGeometryElement.getPointAtLength-01.svg

+<svg xmlns="http://www.w3.org/2000/svg"
+     xmlns:xlink="http://www.w3.org/1999/xlink"
+     xmlns:html="http://www.w3.org/1999/xhtml">
+  <title>SVGGeometryElement.prototype.getPointAtLength clamps its argument to [0, length]</title>
+  <metadata>
+    <html:link rel="help" href="https://svgwg.org/svg2-draft/types.html#__svg__SVGGeometryElement__getPointAtLength"/>
+    <html:meta name="assert" content="SVGGeometryElement.prototype.getPointAtLength clamps its argument."/>
+  </metadata>
+  <g stroke="blue">
+    <line id="line" x1="50" y1="60" x2="100" y2="60"/>
+    <path id="path" d="M40,70L110,70"/>
+  </g>
+  <html:script src="/resources/testharness.js"/>
+  <html:script src="/resources/testharnessreport.js"/>
+  <script><![CDATA[
+  test(function() {
+    let line = document.getElementById('line');
+    let point = line.getPointAtLength(-10);
+    assert_equals(point.x, 50, 'starting x');
+    assert_equals(point.y, 60, 'starting y');
+  }, document.title+', less than zero (SVGLineElement).');
+  test(function() {
+    let path = document.getElementById('path');
+    let point = path.getPointAtLength(-10);
+    assert_equals(point.x, 40, 'starting x');
+    assert_equals(point.y, 70, 'starting y');
+  }, document.title+', less than zero (SVGPathElement).');
+
+  test(function() {
+    let line = document.getElementById('line');
+    assert_less_than(line.getTotalLength(), 80);
+    let point = line.getPointAtLength(80);
+    assert_equals(point.x, 100, 'ending x');
+    assert_equals(point.y, 60, 'ending y');
+  }, document.title+', greater than \'length\' (SVGLineElement).');
+  test(function() {
+    let path = document.getElementById('path');
+    assert_less_than(path.getTotalLength(), 80);
+    let point = path.getPointAtLength(80);
+    assert_equals(point.x, 110, 'ending x');
+    assert_equals(point.y, 70, 'ending y');
+  }, document.title+', greater than \'length\' (SVGPathElement).');
+  ]]></script>
+</svg>

third_party/blink/renderer/core/svg/svg_geometry_element.cc

@@ -141,8 +141,17 @@ SVGPointTearOff* SVGGeometryElement::getPointAtLength(float length) {
   GetDocument().UpdateStyleAndLayoutIgnorePendingStylesheets();
 
   FloatPoint point;
-  if (GetLayoutObject())
-    point = AsPath().PointAtLength(length);
+  if (GetLayoutObject()) {
+    const Path& path = AsPath();
+    if (length < 0) {
+      length = 0;
+    } else {
+      float computed_length = path.length();
+      if (length > computed_length)
+        length = computed_length;
+    }
+    point = path.PointAtLength(length);
+  }
   return SVGPointTearOff::CreateDetached(point);
 }
 

third_party/blink/renderer/core/svg/svg_path_element.cc

@@ -70,7 +70,15 @@ float SVGPathElement::getTotalLength() {
 
 SVGPointTearOff* SVGPathElement::getPointAtLength(float length) {
   GetDocument().UpdateStyleAndLayoutIgnorePendingStylesheets();
-  FloatPoint point = SVGPathQuery(PathByteStream()).GetPointAtLength(length);
+  SVGPathQuery path_query(PathByteStream());
+  if (length < 0) {
+    length = 0;
+  } else {
+    float computed_length = path_query.GetTotalLength();
+    if (length > computed_length)
+      length = computed_length;
+  }
+  FloatPoint point = path_query.GetPointAtLength(length);
   return SVGPointTearOff::CreateDetached(point);
 }