Ensure that a font exists when requesting fontMetrics
Ensure that a font exists when requesting fontMetrics

Add an assert ensuring that primaryFont is not null in Font::fontMetrics
as all callers assume that the returned font object exists and is valid.

BUG=464129
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/1381573003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351460}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk