Fixing tranferFromimageBitmap-toBlob-offscreen.html to not be flaky

Replacing the getElementById by the creation of a new element to ensure
that the test will not be flaky.

Changing also the name of the function to be coherent with the test itself.

Bug: 978554
Change-Id: I8848b18f6d7f6201cce57069c5f4047d4d2f61e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693189
Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677456}

third_party/blink/web_tests/external/wpt/imagebitmap-renderingcontext/tranferFromImageBitmap-ToBlob-offscreen.html

@@ -15,13 +15,13 @@ function testCanvas(ctx, r, g, b, a)
 }
 
 promise_test(function() {
-    function testTransferFromImageBitmapNullability(greenImage) {
+    function testTransferFromImageBitmapToBlobOffscreen(greenImage) {
         var bitmapCanvas = new OffscreenCanvas(width,height);
         var bitmapCtx = bitmapCanvas.getContext('bitmaprenderer');
         bitmapCtx.transferFromImageBitmap(greenImage);
 
         // Make sure the bitmap renderer canvas is filled correctly.
-        var pngImage = document.getElementById("img");
+        var pngImage = new Image();
         bitmapCanvas.convertToBlob().then(function(blob){
           pngImage.src = URL.createObjectURL(blob);
           var myCanvasToTest = document.createElement('canvas');
@@ -43,7 +43,7 @@ promise_test(function() {
     return Promise.all([
         createImageBitmap(greenCanvas),
     ]).then(([greenImage]) => {
-        testTransferFromImageBitmapNullability(greenImage);
+        testTransferFromImageBitmapToBlobOffscreen(greenImage);
     });
 },'Test that transferToBlob works and produce the expected image');