Print Preview cleanup: Remove unnecessary MetricsContext subclasess.

Also convert MetricsContext to ES6 class syntax, and fix newly found linter
errors.

Bug: None
Change-Id: Icc4149d22f5c317d4a28ef633f584c43e7eab0d7
Reviewed-on: https://chromium-review.googlesource.com/c/1379179
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616933}
6 files changed