[DevTools] Accept empty-string timezone IDs

This patch loosens up the timezone ID validation so the form accepts
empty-string inputs.

Bug: chromium:993628, chromium:1015770
Change-Id: I2c0d552dcbc2a3cdf2c31cc129a7ad43b7240955
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868996Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707335}

third_party/blink/renderer/devtools/front_end/sdk/EmulationModel.js

@@ -279,7 +279,8 @@ export class Geolocation {
     // but instead tries to make sense of the input, even for
     // weird-looking timezone IDs. There's not much point in validating
     // the input other than checking if it contains at least one slash.
-    const valid = value.includes('/');
+    // The empty string resets the override, and is accepted as well.
+    const valid = value === '' || value.includes('/');
     return {valid};
   }