[build] Introduce gn arg to disable WebAssembly

For now, that flag only avoid the installation of the 'WebAssembly'
global. Follow-up CLs will remove unneeded code from the binary if the gn
arg is not set to true.

R=ecmziegler@chromium.org, machenbach@chromium.org

Bug: v8:11238, v8:11344
Change-Id: Ie95b608ed12b0476470cb552138e369f863d249b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2646833
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Emanuel Ziegler <ecmziegler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72296}

BUILD.gn

@@ -289,6 +289,11 @@ declare_args() {
   # If enabled then macro definitions that are used in externally visible
   # header files are placed in a separate header file v8-gn.h.
   v8_generate_external_defines_header = false
+
+  # Include support for WebAssembly. If disabled, the 'WebAssembly' global
+  # will not be available, and embedder APIs to generate WebAssembly modules
+  # will fail.
+  v8_enable_webassembly = true
 }
 
 # Derived defaults.
@@ -755,6 +760,9 @@ config("features") {
   if (v8_etw_guid != "") {
     defines += [ "V8_ETW_GUID=\"$v8_etw_guid\"" ]
   }
+  if (v8_enable_webassembly) {
+    defines += [ "V8_ENABLE_WEBASSEMBLY" ]
+  }
 }
 
 config("toolchain") {

src/flags/flag-definitions.h

@@ -782,7 +782,11 @@ DEFINE_BOOL(untrusted_code_mitigations, V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS,
 DEFINE_BOOL(wasm_generic_wrapper, true,
             "allow use of the generic js-to-wasm wrapper instead of "
             "per-signature wrappers")
+#ifdef V8_ENABLE_WEBASSEMBLY
 DEFINE_BOOL(expose_wasm, true, "expose wasm interface to JavaScript")
+#else
+DEFINE_BOOL(expose_wasm, false, "expose wasm interface to JavaScript")
+#endif
 DEFINE_INT(wasm_num_compilation_tasks, 128,
            "maximum number of parallel compilation tasks for wasm")
 DEFINE_DEBUG_BOOL(trace_wasm_native_heap, false,