Check for primaryFont in LayoutTextControl::getAvgCharWidth

Change LayoutTextControl::getAvgCharWidth to check if primaryFont is set
before attempting to access it falling back on measuring the width of 0.

BUG=627818
R=cbiesinger@chromium.org

Review-Url: https://codereview.chromium.org/2149683002
Cr-Commit-Position: refs/heads/master@{#405355}
1 file changed