Add "customPropertiesApplied" metric to StyleResolverStats.

To see this in traces you need to opt into the blink_style
category. This metric will then indicate how many custom
properties are applied for each call to updateStyle.

BUG=694902

Review-Url: https://codereview.chromium.org/2707113003
Cr-Commit-Position: refs/heads/master@{#453862}
3 files changed