feedback: Improve contrast for "Details" header.

The "Details" text in the system & app information dialog was too
low-contrast.

BUG=chromium:1005543
TEST=Took screenshots in linked bug and checked contrast ratios.

Change-Id: Ic0862a00c7ddb3dbbb4e46be8468e2e70a318afc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877648Reviewed-by: J Kardatzke <jkardatzke@chromium.org>
Commit-Queue: Miriam Zimmerman <mutexlox@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709108}

chrome/browser/resources/feedback/css/sys_info.css

@@ -12,6 +12,10 @@ body {
   margin-top: .5em;
 }
 
+#tableTitle {
+  color: #4a4a4a;
+}
+
 #status {
   color: rgb(66, 133, 244);
   display: inline-block;