CL https://codereview.chromium.org/2177273002 changed FastNewFunctionContextStub
to take a number of slots parameter and in-doing so removed the maximum slot
count for FastNewFunctionContextStub. This made it possible to create a
closure which is larger than kMaxRegularHeapObjectSize and so can't be
allocated by FastNewFunctionContextStub.

Reintroduce FastNewFunctionContextStub::kMaxSlots (but make the limit much
larger) to ensure we call the runtime for contexts which need to be
allocated in the LO space.

BUG=chromium:655573

Review-Url: https://codereview.chromium.org/2445703002
Cr-Commit-Position: refs/heads/master@{#40541}