DCHECK style is cacheable in ComputedStyle::CopyNonInheritedFromCached.

In ComputedStyle::CopyNonInheritedFromCached, we copy computed values
of non inherited properties from another ComputedStyle to the current
ComputedStyle. There are several DCHECKS in this function to make
sure that the style we're copying from is cacheable.

This patch replaces those DCHECKS with a single DCHECK on a new
function called MatchedPropertiesCache::IsStyleCacheable.

BUG=628043

Review-Url: https://codereview.chromium.org/2895283003
Cr-Commit-Position: refs/heads/master@{#475267}
3 files changed