Rewrites references to import.meta to a new GetImportMetaObject runtime
call. Embedders can define a callback for creating the meta object using
v8::Isolate::SetHostGetImportMetaObjectCallback. If no callback has been
provided, an empty object with null prototype is created.

This adds an example implementation to d8 that sets meta.url.

Bug: v8:6693
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I6871eec79da45bba81bbbc84b1ffff48534c368d
Reviewed-on: https://chromium-review.googlesource.com/707902
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48433}