Avoid scanning really wide DOMs in textRendererIsNeeded.

Avoiding creation of a Renderer for the text node is a non-essential memory
optimization. So to avoid blowing up on very wide DOMs, this patch limits the
number of siblings to visit to 50.

It improves the page load time of http://alexaing.com/ (slowest parse time in
alexa top million) from 16.4s -> 4.4s on my z620 linux workstation.

BUG=261308

Review URL: https://chromiumcodereview.appspot.com/19652006

git-svn-id: svn://svn.chromium.org/blink/trunk@154722 bbb929c8-8fbe-4397-9dbb-9b2b20218538