Only overrides v8_current_cpu in mac toolchain when declared

Sometimes we expect to build mac target without v8, but there is a
warning that v8_current_cpu is set as a build argument, but not
appeared in declare_args(). If the declaration file v8_target_cpu.gni
is not imported, we should not override it.

Change-Id: I04ef533a565fb9272aa1117ccea1bdee022f5c9a
Reviewed-on: https://chromium-review.googlesource.com/c/1369645
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615530}

AUTHORS

@@ -310,6 +310,7 @@ Halley Zhao <halley.zhao@intel.com>
 Halton Huo <halton.huo@gmail.com>
 Halton Huo <halton.huo@intel.com>
 Hans Hillen <hans.hillen@gmail.com>
+Hao Li <hao.x.li@intel.com>
 Haojian Wu <hokein.wu@gmail.com>
 Hari Singh <hari.singh1@samsung.com>
 Harpreet Singh Khurana <harpreet.sk@samsung.com>

build/toolchain/mac/BUILD.gn

@@ -509,32 +509,44 @@ mac_toolchain("clang_x86") {
 mac_toolchain("clang_x86_v8_arm") {
   toolchain_args = {
     current_cpu = "x86"
-    v8_current_cpu = "arm"
     current_os = "mac"
+
+    if (defined(v8_current_cpu)) {
+      v8_current_cpu = "arm"
+    }
   }
 }
 
 mac_toolchain("clang_x86_v8_mipsel") {
   toolchain_args = {
     current_cpu = "x86"
-    v8_current_cpu = "mipsel"
     current_os = "mac"
+
+    if (defined(v8_current_cpu)) {
+      v8_current_cpu = "mipsel"
+    }
   }
 }
 
 mac_toolchain("clang_x64_v8_arm64") {
   toolchain_args = {
     current_cpu = "x64"
-    v8_current_cpu = "arm64"
     current_os = "mac"
+
+    if (defined(v8_current_cpu)) {
+      v8_current_cpu = "arm64"
+    }
   }
 }
 
 mac_toolchain("clang_x64_v8_mips64el") {
   toolchain_args = {
     current_cpu = "x64"
-    v8_current_cpu = "mips64el"
     current_os = "mac"
+
+    if (defined(v8_current_cpu)) {
+      v8_current_cpu = "mips64el"
+    }
   }
 }