Label canvas tests as slow

These tests have been labeled as slow on Linux and Windows but they are also
timing out on MacOS 10.13.

TBR=serb@chromium.org

Bug: 983642
Change-Id: I70b43867cf47b8056d441a4a3e3399ddb8e3c2e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720877Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681444}

third_party/blink/web_tests/SlowTests

@@ -650,12 +650,9 @@ crbug.com/980804 virtual/scroll_customization/fast/scrolling/scrollbars/mouse-au
 crbug.com/980804 virtual/threaded/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Slow ]
 crbug.com/980804 virtual/main_thread_scrollbar_gestures/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Slow ]
 
-crbug.com/983642 [ Linux ] virtual/gpu/fast/canvas/canvas-composite-alpha.html [ Slow ]
-crbug.com/983642 [ Linux ] virtual/gpu/fast/canvas/canvas-composite-text-alpha.html [ Slow ]
-crbug.com/983642 [ Linux ] virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-e_srgb.html [ Slow ]
-crbug.com/983642 [ Win7 ] virtual/gpu/fast/canvas/canvas-composite-alpha.html [ Slow ]
-crbug.com/983642 [ Win7 ] virtual/gpu/fast/canvas/canvas-composite-text-alpha.html [ Slow ]
-crbug.com/983642 [ Win ] virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-e_srgb.html [ Slow ]
+crbug.com/983642 virtual/gpu/fast/canvas/canvas-composite-alpha.html [ Slow ]
+crbug.com/983642 virtual/gpu/fast/canvas/canvas-composite-text-alpha.html [ Slow ]
+crbug.com/983642 virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-e_srgb.html [ Slow ]
 crbug.com/983642 [ Win ] virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-p3.html [ Slow ]
 crbug.com/983642 [ Win ] virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-rec2020.html [ Slow ]