Now the StringAddStub can optionally convert it's parameters to strings
(following the rules for the addition operator). This could be further
optimized once we have a ToPrimitiveStub, but it should be sufficient
for the moment.

Also removed the unused Strength parameter to the HStringAdd operator,
because string addition does not depend on language mode.

R=jarin@chromium.org
BUG=v8:4307, chromium:532524
LOG=n

Committed: https://crrev.com/d261849e53fbf8c36efae42d478271f87acff70f
Cr-Commit-Position: refs/heads/master@{#30726}

Review URL: https://codereview.chromium.org/1339053002

Cr-Commit-Position: refs/heads/master@{#30858}