[History WebUI] Fix toolbar overflow issue.

Bug: 972967
Change-Id: I72724aa1509386424e2c656d2d3b03cda9752a23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1694247Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676302}

chrome/browser/resources/history/history_toolbar.html

@@ -12,6 +12,7 @@
     <style include="shared-style">
       :host {
         display: flex;
+        overflow: hidden;
         position: relative;
       }