md-history: Fix a search field regression caused by r410495.

The original behavior before r410495 depended on 1px margin-inline-end
specified by the UA stylesheet for -webkit-search-cancel-button. After
r410495, the search field correctly moves the cancel button to the left
side.  This CL recovers the original behavior by adding margin-inline-end
to #searchTerm.

Also, this CL removes selectors for -webkit-search-decoration and
-webkit-search-results-button from cr_toolbar_search_field.html. These
pseudo elements were removed from Blink.

This CL doesn't intend to fix other existing issues.
- We should secure space for the custom clear button at the right side, or
- We should flip the position of the custom clear button; crbug.com/638484

BUG=639256

Change-Id: I9e3368c03067f42235ce4904c47133e4e709a34c
Reviewed-on: https://chromium-review.googlesource.com/c/1491880
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636720}

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html

@@ -74,6 +74,7 @@
         -webkit-font-smoothing: antialiased;
         flex: 1;
         line-height: 185%;
+        margin-inline-end: 2px;
         margin-inline-start: 2px;
         position: relative;
       }
@@ -105,10 +106,9 @@
         width: 100%;
       }
 
-      input[type='search']::-webkit-search-decoration,
-      input[type='search']::-webkit-search-cancel-button,
-      input[type='search']::-webkit-search-results-button {
+      input[type='search']::-webkit-search-cancel-button {
         -webkit-appearance: none;
+        margin: 0;
       }
 
       /** Wide layout. */