Guard build_vars.txt by enable_java_templates

Prevents gn gen failure for v8.

Bug: 1114750, 1115981
Change-Id: I9cdd0ad4e7bc0ed511b3d99d1ec6d004e31e6ba9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355436
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Ben Mason <benmason@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Ben Mason <benmason@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798017}

build/android/BUILD.gn

@@ -123,8 +123,10 @@ _build_vars_json = {
 write_file(build_vars_file, _build_vars_json, "json")
 
 # TODO(agrieve): Update bot recipes to use .json format.
-_lines = [ "final_android_sdk=${_build_vars_json.final_android_sdk}" ]
-if (defined(_build_vars_json.android_secondary_abi_cpu)) {
-  _lines += [ "android_secondary_abi_toolchain=${_build_vars_json.android_secondary_abi_cpu}" ]
+if (enable_java_templates) {
+  _lines = [ "final_android_sdk=${_build_vars_json.final_android_sdk}" ]
+  if (defined(_build_vars_json.android_secondary_abi_cpu)) {
+    _lines += [ "android_secondary_abi_toolchain=${_build_vars_json.android_secondary_abi_cpu}" ]
+  }
+  write_file("$root_build_dir/build_vars.txt", _lines)
 }
-write_file("$root_build_dir/build_vars.txt", _lines)