Fix errorprone javac path for GN.

third_party/errorprone/BUILD.gn

Index: third_party/errorprone/BUILD.gn
diff --git a/third_party/errorprone/BUILD.gn b/third_party/errorprone/BUILD.gn
index 371021b529b15b7f32393060c063c36bf17933d8..9ae64830b38f5c25ca0a864eae137963688bf524 100644
--- a/third_party/errorprone/BUILD.gn
+++ b/third_party/errorprone/BUILD.gn
@@ -43,5 +43,7 @@ java_prebuilt("chromium_errorprone") {
   main_class = "com.google.errorprone.ErrorProneCompiler"
 
   # TODO(mikecase): Remove this extra arg when the bots are on jdk8 or higher.
-  bootclasspath = "$target_gen_dir/javac_java.jar"
+  bootclasspath = rebase_path(

[agrieve, 2016/03/31 00:22:46]

GN should raise errors when paths to non-existent files exist. In this case, I
think it's because java_prebuilt_impl is not adding bootclasspath to its list of
inputs.

Could you also update the template to do so so that this will be caught next
time?

[mikecase (-- gone --), 2016/03/31 18:02:29]

I'm not sure if there is a good way to do this since the jar that is passed in
through bootclasspath is 
generated by errorprone:javac_java and the java_binary_script template where it
is used doesn't depend on that target.

So in order to list bootclasspath as an input in java_binary_script I would need
to...
(1) create a bootclasspath_dep variable and pass it down to java_binary_script
and add it to the
java_binary_script deps.
(2) set bootclasspath_dep to something hacky like 
"//third_party/errorprone:javac_java__process_jar__copy_jar"
because that is what actually lists the jar as an output.

Is there a cleaner way?

[agrieve, 2016/03/31 18:21:52]

The crux of the problem is in the last paragraph of the error message. Running
"gn path" is how to figure this out:

gn path . //third_party/errorprone:chromium_errorprone__java_binary_script
//third_party/errorprone:javac_java__process_jar__copy_jar
//third_party/errorprone:chromium_errorprone__java_binary_script --[private]-->
//third_party/errorprone:chromium_errorprone__build_config --[private]-->
//third_party/errorprone:javac_java --[private]-->
//third_party/errorprone:javac_java__process_jar --[public]-->
//third_party/errorprone:javac_java__process_jar__copy_jar

Showing one of 2 unique non-data paths. 0 of them are public.
Use --all to print all paths.

All of the deps need to be [public]. This is something that our android
templates have been inconsistent about, but it's the correct fix.

I've uploaded what I think the correct fix is here:
https://codereview.chromium.org/1847943003

+          "$root_build_dir/lib.java/third_party/errorprone/javac_java.jar",
+          root_build_dir)
 }