Introduce DOMSVGFactory

This patch provides a new unit tested factory for creating SVG
containers and elements. This code is duplicated twice in the
codebase, but with upcoming changes this would need to be duplicated
even more. Moreover, consolidating this code in one factory allows
us to replace it easily for e.g., supporting Node.js. Therefore, move
this to a central place and update/ES6-ify the related code.

Finally, we replace `setAttributeNS` with `setAttribute` because no
namespace is provided.

test/unit/dom_utils_spec.js

@@ -14,11 +14,72 @@
  */
 
 import {
-  getFilenameFromUrl, isExternalLinkTargetSet, LinkTarget
+  DOMSVGFactory, getFilenameFromUrl, isExternalLinkTargetSet, LinkTarget
 } from '../../src/display/dom_utils';
+import { isNodeJS } from '../../src/shared/util';
 import { PDFJS } from '../../src/display/global';
 
 describe('dom_utils', function() {
+  describe('DOMSVGFactory', function() {
+    let svgFactory;
+
+    beforeAll(function (done) {
+      svgFactory = new DOMSVGFactory();
+      done();
+    });
+
+    afterAll(function () {
+      svgFactory = null;
+    });
+
+    it('`create` should throw an error if the dimensions are invalid',
+        function() {
+      // Invalid width.
+      expect(function() {
+        return svgFactory.create(-1, 0);
+      }).toThrow(new Error('Invalid SVG dimensions'));
+
+      // Invalid height.
+      expect(function() {
+        return svgFactory.create(0, -1);
+      }).toThrow(new Error('Invalid SVG dimensions'));
+    });
+
+    it('`create` should return an SVG element if the dimensions are valid',
+        function() {
+      if (isNodeJS()) {
+        pending('Document is not supported in Node.js.');
+      }
+
+      let svg = svgFactory.create(20, 40);
+
+      expect(svg instanceof SVGSVGElement).toBe(true);
+      expect(svg.getAttribute('version')).toBe('1.1');
+      expect(svg.getAttribute('width')).toBe('20px');
+      expect(svg.getAttribute('height')).toBe('40px');
+      expect(svg.getAttribute('preserveAspectRatio')).toBe('none');
+      expect(svg.getAttribute('viewBox')).toBe('0 0 20 40');
+    });
+
+    it('`createElement` should throw an error if the type is not a string',
+        function() {
+      expect(function() {
+        return svgFactory.createElement(true);
+      }).toThrow(new Error('Invalid SVG element type'));
+    });
+
+    it('`createElement` should return an SVG element if the type is valid',
+        function() {
+      if (isNodeJS()) {
+        pending('Document is not supported in Node.js.');
+      }
+
+      let svg = svgFactory.createElement('svg:rect');
+
+      expect(svg instanceof SVGRectElement).toBe(true);
+    });
+  });
+
   describe('getFilenameFromUrl', function() {
     it('should get the filename from an absolute URL', function() {
       var url = 'http://server.org/filename.pdf';