Fix "XWalkExtensionsIFrameTest.IFrameUsingDocumentWriteShouldNotCrash" test failure

The "XWalkExtensionsIFrameTest.IFrameUsingDocumentWriteShouldNotCrash"
test failed on Linux due to some pending events left after
"xwalk_test_utils::NavigateToURL()". The pending events were
prosessed after the corresponding web contents had been
already destroyed causing the crash.
Merge pull request #1664 from pozdnyakov/Fix_IFrameUsingDocumentWriteShouldNotCrash

"XWalkExtensionsIFrameTest.IFrameUsingDocumentWriteShouldNotCrash" test failure