Bug 1457255 - 3. Turn Java warnings into errors; r=nalexander

Turn Java warnings into errors except for the deprecation and serial
warnings. This is done for all Gradle projects except the thirdparty
project.

build.gradle

@@ -99,7 +99,23 @@ if ('multi' == System.env.AB_CD) {
 }
 
 afterEvaluate {
-    subprojects {
+    subprojects { project ->
+        if (project.name != 'thirdparty') {
+            tasks.withType(JavaCompile) {
+                // Add compiler args for all code except third-party code.
+                options.compilerArgs += [
+                    // Turn on all warnings, except...
+                    "-Xlint:all",
+                    // Deprecation, because we do use deprecated API for compatibility.
+                    "-Xlint:-deprecation",
+                    // Serial, because we don't use Java serialization.
+                    "-Xlint:-serial",
+                    // Turn all remaining warnings into errors,
+                    // unless marked by @SuppressWarnings.
+                    "-Werror"]
+            }
+        }
+
         if (!hasProperty('android')) {
             return
         }