GitLab

A LayoutThemeDefault instance can be associated to a real
WebThemeEngine or a mock WebThemeEngine, which have different
metrics. We need to invalidate a cached value when a WebThemeEngine is
switched.

The bug affects only layout tests.

BUG=673754

Review-Url: https://codereview.chromium.org/2627773002
Cr-Commit-Position: refs/heads/master@{#442799}