Update BrowserTest.TestPopupBounds for non-MD CrOS

Reduce the value of the minimum popup padding
by 1 in BrowserTest.TestPopupBounds. As-is,
the test fails on Chrome for ChromeOS
with --top-chrome-md=non-material since
the toolbar is in fact one pixel shorter
in non-MD than in MD (25px vs. 26px).
See crbug.com/580302 for further context.

BUG=580302

Review URL: https://codereview.chromium.org/1688383004

Cr-Commit-Position: refs/heads/master@{#375479}

chrome/browser/ui/browser_browsertest.cc

@@ -3219,7 +3219,7 @@ IN_PROC_BROWSER_TEST_F(BrowserTest, TestPopupBounds) {
   {
     // Minimum height a popup window should have added to the supplied content
     // bounds when drawn. This accommodates the browser toolbar.
-    const int minimum_popup_padding = 27;
+    const int minimum_popup_padding = 26;
 
     // Creates an untrusted popup window and asserts that the eventual height is
     // padded with the toolbar and title bar height (initial height is content