[FastTextAutosizer] Strip whitespace when computing textLength

This patch switches FastTextAutosizer::textLength to use the text length
without whitespace which more closely matches renderedTextLength().
For example:
<td>[200 whitespace characters]hello</td>

Before this patch this td would have 205 characters and a total
textLength of (205*specifiedFontSize). After this patch this td has 5
characters for a total textLength of (5*specifiedFontSize).

We now pass 1 additional test (clusters-insufficient-text.html) which
covered the above <td> case.

BUG=302005
NOTRY=true

Review URL: https://codereview.chromium.org/142483003

git-svn-id: svn://svn.chromium.org/blink/trunk@165883 bbb929c8-8fbe-4397-9dbb-9b2b20218538