[Do not merge] Avoid SVG null transform error

If we
  git checkout c05aaf865babd596bfa824825e3d594ebc9a2372
and open
  test/testcases/auto-test-svg-transform.html
the console reported
  Error: Invalid value for <rect> attribute transform="null" web-animations.js:5276
  Error: Invalid value for <rect> attribute transform="null" bootstrap.js:331
  Error: Invalid value for <rect> attribute transform="null" bootstrap.js:331

With this CL, we now avoid the error in web-animations.js:5276

The error could also be avoided by changing line 5307 to
  target._bases = {transform: ''}; // Avoids transform being set to null

[Do not merge as c05aaf865babd596bfa824825e3d594ebc9a2372 was reverted]