Add a Mac10.14 web test failure to expectations

Add external/wpt/css/css-color/color-initial-canvastext.html to
TestExpectations for MacOS 10.14.

TBR=wangxianzhu@chromium.org

Bug: 1047472, 1028242
Change-Id: I76c6d24475b6c08fa5a97cea5e9f729026c43e91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032168Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737102}

third_party/blink/web_tests/TestExpectations

@@ -6511,3 +6511,6 @@ crbug.com/1047208 virtual/omt-service-worker-startup/http/tests/serviceworker/up
 crbug.com/1047208 http/tests/serviceworker/update-served-from-cache.html [ Pass Failure ]
 crbug.com/1047293 [ Mac ] editing/pasteboard/pasteboard_with_unfocused_selection.html [ Pass Failure ]
 crbug.com/1047420 [ Mac ] http/tests/devtools/persistence/persistence-move-breakpoints-on-reload.js [ Pass Failure ]
+
+# Color difference on MacOS 10.14
+crbug.com/1047126 [ Mac10.14 ] external/wpt/css/css-color/color-initial-canvastext.html [ Failure ]