Use relative paths in gn rules to avoid build issues in tripple headed golem build

Change-Id: Ib0b045a5a7c78d69ccd1aa2150a80d9665d8c90a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/507400
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Jonas Jensen <jonasfj@google.com>

utils/dartpad/compile_wasm.gni

@@ -82,8 +82,8 @@ template("compile_dart2wasm") {
     }
 
     deps = [
-      "//utils/dart2wasm:compile_dart2wasm_platform",
-      "//utils/dart2wasm:dart2wasm_snapshot",
+      "../dart2wasm:compile_dart2wasm_platform",
+      "../dart2wasm:dart2wasm_snapshot",
     ]
     if (product_mode) {
       deps += [ "//third_party/binaryen:wasm-opt" ]