Resolve references to "this" the same way as normal variables

Resolve references to "this" the same way as normal variables

Make the parser handle references to "this" as unresolved variables, so the
same logic as for the rest of function parameters is used for the receiver.
Minor additions to the code generation handle copying the receiver to the
context, along with the rest of the function parameters.

Based on work by Adrian Perez de Castro <aperez@igalia.com>.

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1097283003

Cr-Commit-Position: refs/heads/master@{#28236}