Rebind ChromeVox incremental search to ctrl+f.

While we have an inaccessible browser find, let ChromeVox take care of find using its incremental search.

TEST=ctrl+f brings up i-search.

Review URL: https://codereview.chromium.org/1807843005

Cr-Commit-Position: refs/heads/master@{#381738}

chrome/browser/resources/chromeos/chromevox/chromevox/background/keymaps/next_keymap.json

@@ -496,9 +496,9 @@
     {
       "command": "toggleSearchWidget",
       "sequence": {
-        "cvoxModifier": true,
         "keys": {
-          "keyCode": [191]
+          "keyCode": [70],
+          "ctrlKey": [true]
         }
       }
     }