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It currently just uses 1.0. Instead, fetch it with
ui::GetScaleFactorForNativeView and invalidate it when the scale factor
changes.

This is a small fix for a simple HiDPI bug, but eventually we will be
able to scrap CachedLabel: The performance improvement it offers will be
redundant with http://crbug.com/240037.

BUG=368493

Review URL: https://codereview.chromium.org/405373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284649 0039d316-1c4b-4281-b951-d872f2087c98