Add a variable use_system_xcode to GN.

build/toolchain/mac/BUILD.gn

Index: build/toolchain/mac/BUILD.gn
diff --git a/build/toolchain/mac/BUILD.gn b/build/toolchain/mac/BUILD.gn
index 0c00e995fffe139ac39badd12790acbad3a78a7a..2bb96c75cbeb333f20ab89a7243874fedc99099c 100644
--- a/build/toolchain/mac/BUILD.gn
+++ b/build/toolchain/mac/BUILD.gn
@@ -62,6 +62,12 @@ tool_versions =
 # build args in this definition.
 template("mac_toolchain") {
   toolchain(target_name) {
+    if (use_system_xcode) {
+      env_wrapper = ""
+    } else {
+      env_wrapper = "export DEVELOPER_DIR=$hermetic_xcode_path; "
+    }
+
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     assert(defined(invoker.toolchain_args),
@@ -212,7 +218,7 @@ template("mac_toolchain") {
     tool("alink") {
       script =
           rebase_path("//build/toolchain/mac/filter_libtool.py", root_build_dir)
-      command = "rm -f {{output}} && TOOL_VERSION=${tool_versions.filter_libtool} python $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
+      command = "$env_wrapper rm -f {{output}} && TOOL_VERSION=${tool_versions.filter_libtool} python $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
       description = "LIBTOOL-STATIC {{output}}"
       outputs = [
         "{{output_dir}}/{{target_output_name}}{{output_extension}}",
@@ -250,7 +256,7 @@ template("mac_toolchain") {
       replace_command = "if ! cmp -s \"$temporary_tocname\" \"$tocname\"; then mv \"$temporary_tocname\" \"$tocname\""
       extract_toc_command = "{ otool -l \"$dylib\" | grep LC_ID_DYLIB -A 5; nm -gP \"$dylib\" | cut -f1-2 -d' ' | grep -v U\$\$; true; }"
 
-      command = "if $does_reexport_command ; then $link_command && $extract_toc_command > \"$tocname\"; else $link_command && $extract_toc_command > \"$temporary_tocname\" && $replace_command ; fi; fi"
+      command = "$env_wrapper if $does_reexport_command ; then $link_command && $extract_toc_command > \"$tocname\"; else $link_command && $extract_toc_command > \"$temporary_tocname\" && $replace_command ; fi; fi"
 
       rspfile_content = "{{inputs_newline}}"
 
@@ -291,7 +297,7 @@ template("mac_toolchain") {
       rspfile = sofile + ".rsp"
       pool = "//build/toolchain:link_pool($default_toolchain)"
 
-      link_command = "$linker_driver $ld -bundle {{ldflags}} -o \"$sofile\" -Wl,-filelist,\"$rspfile\""
+      link_command = "$env_wrapper $linker_driver $ld -bundle {{ldflags}} -o \"$sofile\" -Wl,-filelist,\"$rspfile\""
       if (is_component_build) {
         link_command += " -Wl,-install_name,@rpath/{{target_output_name}}{{output_extension}}"
       }
@@ -334,7 +340,7 @@ template("mac_toolchain") {
       # do for command-line arguments. Thus any source names with spaces, or
       # label names with spaces (which GN bases the output paths on) will be
       # corrupted by this process. Don't use spaces for source files or labels.
-      command = "$linker_driver $ld $dsym_switch {{ldflags}} -o \"$outfile\" -Wl,-filelist,\"$rspfile\" {{solibs}} {{libs}}"
+      command = "$env_wrapper $linker_driver $ld $dsym_switch {{ldflags}} -o \"$outfile\" -Wl,-filelist,\"$rspfile\" {{solibs}} {{libs}}"
       description = "LINK $outfile"
       rspfile_content = "{{inputs_newline}}"
       outputs = [
@@ -400,7 +406,7 @@ template("mac_toolchain") {
         _sdk_name = mac_sdk_name
         _min_deployment_target = mac_deployment_target
       }
-      command = "rm -f {{output}} && " +
+      command = "$env_wrapper rm -f {{output}} && " +
                 "TOOL_VERSION=${tool_versions.compile_xcassets} " +
                 "python $_tool -p $_sdk_name -t $_min_deployment_target " +
                 "-T {{bundle_product_type}} -o {{output}} {{inputs}}"