WebTestProxy should explode if readback requests are unhandled

content/shell/renderer/test_runner/web_test_proxy.cc

Index: content/shell/renderer/test_runner/web_test_proxy.cc
diff --git a/content/shell/renderer/test_runner/web_test_proxy.cc b/content/shell/renderer/test_runner/web_test_proxy.cc
index 0342bdd637a5d5e851793659cbe715e8d55a3099..bfecc736a5fe2cdcb0331d515afefae47d993021 100644
--- a/content/shell/renderer/test_runner/web_test_proxy.cc
+++ b/content/shell/renderer/test_runner/web_test_proxy.cc
@@ -319,6 +319,9 @@ WebTestProxyBase::WebTestProxyBase()
 
 WebTestProxyBase::~WebTestProxyBase() {
   test_interfaces_->windowClosed(this);
+  // Tests must wait for readback requests to finish before notifying that
+  // they are done.
+  CHECK_EQ(0u, composite_and_readback_callbacks_.size());
 }
 
 void WebTestProxyBase::SetInterfaces(WebTestInterfaces* interfaces) {