Disable flaky canvas layout tests

Disables Mac Debug:
virtual/disable-deferred-rendering/fast/canvas/color-space/canvas-drawImage-offscreenCanvas.html
virtual/disable-deferred-rendering/fast/canvas/OffscreenCanvas-placeholder-createImageBitmap.html

TBR=fserb

Bug: 1010472, 1012146
Change-Id: I8e1d2c28ee1710267293e5576a968617a45f1c12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856164
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705103}

third_party/blink/web_tests/TestExpectations

@@ -5797,3 +5797,5 @@ crbug.com/1013391 http/tests/devtools/a11y-axe-core/performance/performance-pane
 crbug.com/1013391 http/tests/devtools/a11y-axe-core/settings/shortcuts-a11y-test.js [ Timeout ]
 crbug.com/1013523 [ Release ] virtual/cross-origin-embedder-policy/external/wpt/html/cross-origin-embedder-policy/require-corp.https.html [ Failure Pass ]
 crbug.com/1013523 [ Release Linux Mac ] external/wpt/html/cross-origin-embedder-policy/require-corp.https.html [ Failure Pass ]
+crbug.com/1010472 [ Mac Debug ] virtual/disable-deferred-rendering/fast/canvas/color-space/canvas-drawImage-offscreenCanvas.html [ Failure Pass ]
+crbug.com/1010472 [ Mac Debug ] virtual/disable-deferred-rendering/fast/canvas/OffscreenCanvas-placeholder-createImageBitmap.html [ Failure Pass ]