Policy webUI: set minimum width for policy search field

Bug: 1045887
Change-Id: Iee367ff529aaa268aa0a558f13c3727c29f67880
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036397Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Yann Dago <ydago@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738185}

components/policy/resources/webui/policy.css

@@ -135,11 +135,15 @@ body > header {
   height: 56px;
 }
 
-.sshow-unset-checkbox,
+.show-unset-checkbox,
 .search-field-container {
   margin-inline-start: auto;
 }
 
+.search-field-container {
+  flex: 0 1 200px;
+}
+
 .no-policy:not([hidden]) {
   display: flex;
   justify-content: center;