One caveat: the Crankshaft stub used to preserve callee-clobbered double
registers, which is contrary to any real platform ABI that we support. Since the
only current use of this stub is in Crankshaft, the instruction there now must
be marked as double-clobbering. This might result in a small performance
regression. However, when this stub is eventually used in TF-generated code, it
will be called from deferred code that can save doubles only on the rarely-taken
path... something that Crankshaft can't do.

BUG=chromium:608675

Review-Url: https://codereview.chromium.org/2206333003
Cr-Commit-Position: refs/heads/master@{#38371}