Add SourceMapObjectParserJs.getLineCount implementation

It was missing but used in the no JVM version when building source maps.
The implementation just returns the value of the lineCount property.

Fixes google#2826
Closes google#2829

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187063993

src/com/google/debugging/sourcemap/SourceMapObjectParserJs.java

@@ -21,6 +21,7 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 import jsinterop.annotations.JsMethod;
+import jsinterop.annotations.JsOverlay;
 import jsinterop.annotations.JsPackage;
 import jsinterop.annotations.JsType;
 
@@ -56,8 +57,11 @@ private static class JsonMap {
     Section[] sections;
     String[] sources;
     String[] names;
-    @JsMethod
-    native Object getLineCount();
+
+    @JsOverlay
+    public final Object getLineCount() {
+      return this.lineCount;
+    }
   }
 
   @JsType(isNative = true, name = "Object", namespace = JsPackage.GLOBAL)