Like CSP flag 'unsafe-eval', which communicates if both JS source
files and WASM binary files may be compiled, this CL adds a similar
flag for the compilation of WASM binary files.

That is, a WASM binary file will be compiled only if the new flag is
defined, or the flag for 'unsafe-eval' allows it. These flags are
implemented as callback functions on the isolate. The callbacks get a
(CSP) context, and a string, and returns the corresponding value of
the flag.

Both callbacks are initialized with the nullptr, and is used to
communicate that no CSP policy is defined. This allows this concept to
work, independent of it running in Chrome.

It also does a small clean up in api.cc to use macro CALLER_SETTERS,
instead of explicit code when appropriate.

Bug: v8:7041
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Idb3356574ae2a298057e6b7bccbd3492831952ae
Reviewed-on: https://chromium-review.googlesource.com/759162Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49243}