This CL splits CodeGenerator::GenerateCode into two new functions:
AssembleCode and FinishCodeObject. AssembleCode does not access or
modify the JS heap, which means that AssembleCode can be executed on
background threads. FinishCodeObject allocates the generated code object
on the JS heap and therefore has to be executed on the main thread.

Implementation details:
The GenerateCode function has been split just before out-of-line code is
assembled. The reason is that code stubs may be generated when
out-of-line code is assembled, which potentially allocates these code
stubs on the heap.

- Parts of initialization of the CodeGenerator has been moved from the
constructor to an Initialize function so that we can instantiate an empty
CodeGenerator object in PipelineData.

R=bmeurer@chromium.org, mstarzinger@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2229243003
Cr-Commit-Position: refs/heads/master@{#38604}