Migrate the Object.keys builtin to the CodeStubAssembler and
use the enum cache backing store whenever it is available. This
gives a nice speedup of 1.5x to 2x when using Object.keys on fast-mode
objects that have (or can have) an enum cache.

R=cbruni@chromium.org
BUG=v8:5269,v8:6405

Review-Url: https://codereview.chromium.org/2853393002
Cr-Commit-Position: refs/heads/master@{#45361}