When running main-thread compiler-dispatcher jobs, ensure that we enter the
correct Context. Also adds a test for compiling an extension in the compiler
dispatcher to ensure that idle tasks enter the correct context before
finalizing the compilation.

BUG=v8:5203

Review-Url: https://codereview.chromium.org/2679193004
Cr-Commit-Position: refs/heads/master@{#43111}