Add expansion of platform mitigations to chrome://sandbox

This takes the values returned as base::Value from the sandbox
policy and allows them to be expanded in the summary display of
chrome://sandbox.

Expansion is deferred as the page can become slow in debug builds if
it is performed every time, and expanded values are not always
required for debugging.

Bug: 1115305
Change-Id: I96d71b21ccc28e678fdb3fb9714f30cf88b730ef
Tests: manual testing on Win7 x86 and Win10 10.0.18362
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350331Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797783}

chrome/browser/resources/sandbox_internals/sandbox_internals.html

@@ -12,6 +12,7 @@
       #sandbox-status td {
         border: 1px solid gray;
         padding: 3px;
+        vertical-align: top;
       }
       #evaluation {
         font-weight: bold;
@@ -28,6 +29,16 @@
       .info {
         background-color: rgb(169, 217, 239);
       }
+<if expr="is_win">
+      .expander {
+        display: inline;
+        padding-right: 1em;
+      }
+      .mitigations {
+        display: inline;
+        font-family: monospace;
+      }
+</if>
     </style>
     <script src="chrome://resources/js/cr.js"></script>
 <if expr="is_linux">