Add dark-mode-default virtual tests for images.

Add virtual tests for default dark mode behavior for images. This will
help check and correct default behavior for images.

Bug: 1116914
Change-Id: Ifcd6db9df15b5616511fdc50125f277071341976
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434023
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Prashant Nevase <prashant.n@samsung.com>
Cr-Commit-Position: refs/heads/master@{#811931}

third_party/blink/web_tests/VirtualTestSuites

@@ -262,7 +262,8 @@
   },
   {
     "prefix": "dark-mode-default",
-    "bases": ["dark-mode/colors"],
+    "bases": ["dark-mode/colors",
+              "dark-mode/images"],
     "args": ["--blink-settings=forceDarkModeEnabled=true"]
   },
   {

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

@@ -0,0 +1,3 @@
+# This suite runs the tests in web_tests/dark-mode/images
+# with --blink-settings=forceDarkModeEnabled=true
+# See the virtual_test_suites() method in tools/blinkpy/web_tests/port/base.py.