[gn] Make the trap handler API header available on all platforms

We extracted the platform-specific API header into separate, platform-
specific header files to avoid OS-specific includes in include/v8.h.
As the platform-specific headers are only needed on their specific
platform, we did not make them available on other platforms in BUILD.gn.
Unfortunately, this is not possible, because the `include` validation in
`gn gen --check` checks the availability of header files without
considering `#ifdef` in the source files. Therefore the check fails on
Windows when the POSIX-specific header file is not available.

With this CL platform-specific headers will be generally available. This
is no problem because they can only be included on their specific
platform anyways.

R=adamk@chromium.org

Bug: chromium:921971
Change-Id: Iee7cc22063ce31a536f937d73799b7217ac1a94e
Reviewed-on: https://chromium-review.googlesource.com/c/1443057Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59196}

BUILD.gn

@@ -1470,13 +1470,13 @@ v8_header_set("v8_headers") {
     "include/v8config.h",
   ]
 
-  if (is_linux || is_mac) {
-    sources += [ "include/v8-wasm-trap-handler-posix.h" ]
-  }
-
-  if (is_win) {
-    sources += [ "include/v8-wasm-trap-handler-win.h" ]
-  }
+  sources += [
+    # The following headers cannot be platform-specific. The include validation
+    # of `gn gen $dir --check` requires all header files to be available on all
+    # platforms.
+    "include/v8-wasm-trap-handler-posix.h",
+    "include/v8-wasm-trap-handler-win.h",
+  ]
 
   deps = [
     ":v8_version",