Fix references to high contrast mode missed in last CL.

Bug: 938511
Change-Id: I52a019e87c3a6772d0c3d3fc973876f66a0d275b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532886
Auto-Submit: Aran Gilman <gilmanmh@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#642702}

third_party/blink/web_tests/virtual/dark-mode/paint/dark-mode/page-policy-all/README.txt

 # This suite runs the tests in LayoutTests/paint/dark-mode
-# with --blink-settings="highContrastMode=3,darkModePagePolicy=0"
+# with --blink-settings="darkMode=3,darkModePagePolicy=0"
 # See the virtual_test_suites() method in tools/blinkpy/web_tests/port/base.py.

third_party/blink/web_tests/virtual/dark-mode/paint/dark-mode/page-policy-background/README.txt

 # This suite runs the tests in LayoutTests/paint/dark-mode
-# with --blink-settings="highContrastMode=3,darkModePagePolicy=1"
+# with --blink-settings="darkMode=3,darkModePagePolicy=1"
 # See the virtual_test_suites() method in tools/blinkpy/web_tests/port/base.py.