De-flake two lazyload WPT tests.

They should only set the src of an img after registering the onload
handler, to avoid the onload handler missing the load event.

See also discussion here about the flakiness this is fixing:
https://github.com/web-platform-tests/wpt/pull/22375

Change-Id: I78707beb1cd2ce609d948dcc1e5b22a6e3b1b58b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2127524Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754577}

third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/the-img-element/image-loading-lazy-slow-aspect-ratio.html

@@ -4,7 +4,7 @@
   <link rel="author" title="Chris Harrelson" href="mailto:chrishtr@chromium.org">
   <link rel="help" href="https://html.spec.whatwg.org/multipage/#the-img-element">
   <script src="/common/reftest-wait.js"></script>
-  <img id=target loading="lazy" src="resources/image.png"
+  <img id=target loading="lazy"
       width="200" height="100" style="width: 100px; height: auto; border: 1px solid black">
 <script>
   let loaded = false;
@@ -17,4 +17,5 @@
       takeScreenshot();
     }));
   };
+  target.src = "resources/image.png";
 </script>

third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/the-img-element/image-loading-lazy-slow.html

@@ -5,7 +5,7 @@
   <link rel="help" href="https://html.spec.whatwg.org/multipage/#the-img-element">
 
   <script src="/common/reftest-wait.js"></script>
-  <img id=target loading="lazy" src="resources/image.png"
+  <img id=target loading="lazy"
        width="330" height="254" style="border: 1px solid black">
 <script>
   let loaded = false;
@@ -18,4 +18,5 @@
       takeScreenshot();
     }));
   };
+  target.src = "resources/image.png";
 </script>