Fix racy condition in case of multiple parallel screenshots

Page.screenshot operates the global state of the page. In case of
multiple Page.screenshot() commands running in parallel with different
clipping rects, they interfere with each other.

This patch makes Page.screenshot() commands run sequencially
even though they were called in parallel.

Fixes #15.

test/goldentest.js

@@ -70,6 +70,23 @@ describe('GoldenTests', function() {
             }
         });
     });
+
+    imageTest('screenshot-parallel-calls.png', async function() {
+        await page.setViewportSize({width: 500, height: 500});
+        await page.navigate(STATIC_PREFIX + '/grid.html');
+        var promises = [];
+        for (var i = 0; i < 3; ++i) {
+            promises.push(page.screenshot({
+                clip: {
+                    x: 50 * i,
+                    y: 0,
+                    width: 50,
+                    height: 50
+                }
+            }));
+        }
+        return await promises[1];
+    });
 });
 
 /**