Only use fontMetrics if available in line-height computation

Change ComputedStyle::computedLineHeight to check if primaryFont is non
null before calling fontMetrics().lineSpacing() as metrics requires the
font to have been loaded. This is a speculative fix for a top crasher.

BUG=542338
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/1410173002

Cr-Commit-Position: refs/heads/master@{#354686}
1 file changed