Reporting top 3 lines of stack for js errors

This should help investigating js errors. It includes line, column
and names of two functions who calls method with js error.

Change-Id: Icc893161e9c9bb0cde4ec2aecd96fe5e71d780f3

polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js

@@ -116,7 +116,13 @@
     if (error) {
       line = line || error.lineNumber;
       column = column || error.columnNumber;
-      msg = msg || error.toString();
+      let shortenedErrorStack = msg;
+      if (error.stack) {
+        const errorStackLines = error.stack.split('\n');
+        shortenedErrorStack = errorStackLines.slice(0,
+            Math.min(3, errorStackLines.length)).join('\n');
+      }
+      msg = shortenedErrorStack || error.toString();
     }
     const payload = {
       url,

polygerrit-ui/app/elements/core/gr-reporting/gr-reporting_test.html

@@ -341,6 +341,7 @@ limitations under the License.
 
       test('is reported', () => {
         const error = new Error('bar');
+        error.stack = undefined;
         emulateThrow('bar', 'http://url', 4, 2, error);
         assert.isTrue(reporter.calledWith('error', 'exception', 'bar'));
         const payload = reporter.lastCall.args[3];
@@ -352,6 +353,15 @@ limitations under the License.
         });
       });
 
+      test('is reported with 3 lines of stack', () => {
+        const error = new Error('bar');
+        emulateThrow('bar', 'http://url', 4, 2, error);
+        const expectedStack = error.stack.split('\n').slice(0, 3)
+            .join('\n');
+        assert.isTrue(reporter.calledWith('error', 'exception',
+            expectedStack));
+      });
+
       test('prevent default event handler', () => {
         assert.isTrue(emulateThrow());
       });