Allow WebApk.apk to use custom proguard_jar_path

Change-Id: Iaa78ac6b6ad9050479c0e326e36c80cabb9800f3
Reviewed-on: https://chromium-review.googlesource.com/1231386Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592143}

chrome/android/webapk/shell_apk/BUILD.gn

@@ -92,6 +92,9 @@ template("webapk_tmpl") {
     native_lib_placeholders = [ "libfoo.so" ]
 
     if (!is_java_debug) {
+      if (defined(webapk_proguard_jar_path)) {
+        proguard_jar_path = webapk_proguard_jar_path
+      }
       proguard_enabled = true
       proguard_configs = [
         "//chrome/android/webapk/shell_apk/proguard.flags",