Update dark mode default thresholds

Values from manual testing.

Bug: 996227
Change-Id: I854f1690f039842c21d97e9324c581156cfa7939
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761358Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689508}

third_party/blink/renderer/core/frame/settings.json5

@@ -919,13 +919,13 @@
     },
     {
       name: "darkModeTextBrightnessThreshold",
-      initial: "256",
+      initial: "150",
       type: "int",
       invalidate: "Paint",
     },
     {
       name: "darkModeBackgroundBrightnessThreshold",
-      initial: "0",
+      initial: "205",
       type: "int",
       invalidate: "Paint",
     },

third_party/blink/web_tests/VirtualTestSuites

@@ -439,47 +439,47 @@
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/grayscale-images",
-    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=0,darkModeImageStyle=1"]
+    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=0,darkModeImageStyle=1,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/image-filter-all",
-    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=0"]
+    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=0,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/image-filter-none",
-    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=1"]
+    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=1,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/image-filter-smart",
-    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=2"]
+    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=2,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/native-theme-off",
-    "args": ["--blink-settings=darkMode=3"]
+    "args": ["--blink-settings=darkMode=3,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/native-theme-on",
-    "args": ["--blink-settings=darkMode=3"]
+    "args": ["--blink-settings=darkMode=3,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/page-policy-all",
-    "args": ["--blink-settings=darkMode=3,darkModePagePolicy=0"]
+    "args": ["--blink-settings=darkMode=3,darkModePagePolicy=0,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/page-policy-background",
-    "args": ["--blink-settings=darkMode=3,darkModePagePolicy=1"]
+    "args": ["--blink-settings=darkMode=3,darkModePagePolicy=1,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "dark-mode",
     "base": "paint/dark-mode/svg-invert-all",
-    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=0"]
+    "args": ["--blink-settings=darkMode=3,darkModeImagePolicy=0,darkModeTextBrightnessThreshold=256,darkModeBackgroundBrightnessThreshold=0"]
   },
   {
     "prefix": "presentation",