Correct dark-mode-images-grayscale virtual tests.

While organizing the virtual tests for dark mode images in grayscale,
common configurations were removed. But as earlier code did not have
grayscale argument explicitly set, the results were same as default
dark mode. This patch corrects the argument needed for grayscale and
now expected image results look grayish.

Bug: 1116914
Change-Id: Iddf66c043d754ee6ed1607ca5c6f9908afbed1ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434191Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Prashant Nevase <prashant.n@samsung.com>
Cr-Commit-Position: refs/heads/master@{#811538}

third_party/blink/web_tests/VirtualTestSuites

@@ -279,7 +279,7 @@
   {
     "prefix": "dark-mode-images-grayscale",
     "bases": ["dark-mode/images"],
-    "args": ["--blink-settings=forceDarkModeEnabled=true,forceDarkModeImagePolicy=0,forceDarkModeImageStyle=1"]
+    "args": ["--blink-settings=forceDarkModeEnabled=true,forceDarkModeImagePolicy=0,forceDarkModeImageGrayscale=1.0"]
   },
   {
     "prefix": "presentation",

third_party/blink/web_tests/virtual/dark-mode-images-grayscale/dark-mode/images/README.txt

 # This suite runs the tests in web_tests/dark-mode/images with
-# --blink-settings=forceDarkModeEnabled=true,forceDarkModeImagePolicy=0,forceDarkModeImageStyle=1
+# --blink-settings=forceDarkModeEnabled=true,forceDarkModeImagePolicy=0,forceDarkModeImageGrayscale=1.0
 # See the virtual_test_suites() method in tools/blinkpy/web_tests/port/base.py.