[DevTools] Use empty string `defaultValue`s for CSS media features

Explicitly passing `'no-preference'` as a CSS media feature value has
different semantics than passing the empty string (which clears the
emulation override), causing difficult-to-debug issues.

Bug: chromium:1011722, chromium:1004246
Change-Id: I371681ba646306d4f5e8bff10e8c502329a87093
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1844816Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703255}

third_party/blink/renderer/devtools/front_end/sdk/module.json

@@ -273,12 +273,12 @@
             "settingName": "emulatedCSSMediaFeaturePrefersColorScheme",
             "settingType": "enum",
             "storageType": "session",
-            "defaultValue": "no-preference",
+            "defaultValue": "",
             "options": [
                 {
                     "title": "Do not emulate CSS prefers-color-scheme",
                     "text": "No emulation",
-                    "value": "no-preference"
+                    "value": ""
                 },
                 {
                     "title": "Emulate CSS prefers-color-scheme: light",
@@ -300,12 +300,12 @@
           "settingName": "emulatedCSSMediaFeaturePrefersReducedMotion",
           "settingType": "enum",
           "storageType": "session",
-          "defaultValue": "no-preference",
+          "defaultValue": "",
           "options": [
               {
                   "title": "Do not emulate CSS prefers-reduced-motion",
                   "text": "No emulation",
-                  "value": "no-preference"
+                  "value": ""
               },
               {
                   "title": "Emulate CSS prefers-reduced-motion: reduce",