Wait for html to load before selecting node.

TestRunner.loadHTML() does not wait for the document to load or even
fully parse before returning. When the html contains a parser blocking
stylesheet in a link element, the parsing is typically not finished when
loadHTML returns.

When BlockHTMLParserOnStyleSheets is enabled, this test started failing
because we were trying to find a node which were not yet parsed.

Bug: 891767
Change-Id: I4d6012a732143baf93c67a5e6cc01a78da617d4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023892
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735974}

third_party/blink/web_tests/http/tests/devtools/elements/styles-4/styles-source-offsets.js

@@ -58,6 +58,17 @@ body.mainpage {
     }
   }
 
+  await (() => {
+    // TODO(crbug.com/1046354): This is a workaround for loadHTML() resolving
+    // before parsing is finished. In this case the stylesheet is blocking html
+    // parsing with BlockHTMLParsingOnStyleSheets enabled and mainBody is not
+    // found by selectNodeWithId below.
+    let resolve;
+    const promise = new Promise(r => resolve = r);
+    TestRunner.waitForPageLoad(() => resolve());
+    return promise;
+  })();
+
   ElementsTestRunner.selectNodeWithId('mainBody', step1);
 
   async function step1(node) {