The resume trampolin used to call the generator function with the context of the
last suspension rather than the closure's context.  While that was fine for
Ignition, Turbofan got utterly confused.  With this CL, the resume trampolin
always passes in the closure's context (like in the very first call of the
generator function).  The generator function itself then restores its previously
current context by reading it from the generator object and doing a
PushContext.

BUG=chromium:681171

Review-Url: https://codereview.chromium.org/2639533002
Cr-Commit-Position: refs/heads/master@{#42407}