Split generation of dawncpp headers from libdawncpp

libdawn_native also requires the dawncpp headers. When we separated
libdawn_cpp from the dawn_headers, libdawn_native lost the cpp headers
and caused a compile failure. This patch separates the dawncpp headers
as a separate target so both libdawncpp and libdawn_native can depend
on them.

Bug: dawn:22
Change-Id: I4172f1654377afac8c4314123ee8b5b81dc7c928
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12300
Reviewed-by: David Turner <digit@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn/BUILD.gn

@@ -50,6 +50,27 @@ source_set("dawn_headers") {
   ]
 }
 
+###############################################################################
+# Dawn C++ headers
+###############################################################################
+
+dawn_json_generator("dawncpp_headers_gen") {
+  target = "dawncpp_headers"
+  outputs = [
+    "src/include/dawn/dawncpp.h",
+  ]
+}
+
+source_set("dawncpp_headers") {
+  public_deps = [
+    ":dawn_headers",
+    ":dawncpp_headers_gen",
+  ]
+
+  sources = get_target_outputs(":dawncpp_headers_gen")
+  sources += [ "${dawn_root}/src/include/dawn/EnumClassBitmasks.h" ]
+}
+
 ###############################################################################
 # Dawn C++ wrapper
 ###############################################################################
@@ -58,17 +79,15 @@ dawn_json_generator("dawncpp_gen") {
   target = "dawncpp"
   outputs = [
     "src/dawn/dawncpp.cpp",
-    "src/include/dawn/dawncpp.h",
   ]
 }
 
 source_set("dawncpp") {
   deps = [
-    ":dawn_headers",
     ":dawncpp_gen",
+    ":dawncpp_headers",
   ]
   sources = get_target_outputs(":dawncpp_gen")
-  sources += [ "${dawn_root}/src/include/dawn/EnumClassBitmasks.h" ]
 }
 
 ###############################################################################