Use a composed tree based traversal in computing quads in LinkHighlight::computeQuads().

This is a follow-up patch for https://codereview.chromium.org/448593002 to fix a crash.

The previous patch uses renderer.slowFirstChild() to get an child renderer in computeQuads(). 
However, this is not always correct. For example, suppose that <a><div></div></a> is given, the render tree would be:

  RenderBlock (anonymous) 0x6110004a6400
    RenderInline 0x60c00014e6c0 A       0x60d00006cf10
  RenderBlock (anonymous) 0x6110004a6180
    RenderBlock 0x6110004a62c0  DIV     0x60b00004d930 CLASS="_muf"

In this case, there is no child renderer for RenderInline object(0x60c00014e6c0).

Instead, LinkHighlight::computeQuads() should use a composed tree based traversal to compute quads correctly because we've already used a composed tree based traversal to get an parent object in the previous patch.

BUG=417191

Review URL: https://codereview.chromium.org/622253004

git-svn-id: svn://svn.chromium.org/blink/trunk@183756 bbb929c8-8fbe-4397-9dbb-9b2b20218538