[DevTools] Improve contrast of "This page is not secure" overview text

Updates color from yellow to black (by relying on default color)
to improve contrast and better match neutral state icon color.

Screenshot: https://drive.google.com/file/d/1JMptH0qC9cIyPIzSD99T29Qvm22M4fr-/view?usp=sharing

Bug: 900027
Change-Id: I1732de356aed28c4c505b5b24cf0e3c666b927fb
Reviewed-on: https://chromium-review.googlesource.com/c/1321858
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Reviewed-by: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606265}

third_party/blink/renderer/devtools/front_end/security/mainView.css

@@ -87,7 +87,6 @@
 
     background: #fff;
     border-color: rgb(217, 217, 217);
-    color: rgba(0, 0, 0, 36);
 }
 
 .security-summary-secure .triangle-pointer,
@@ -96,11 +95,6 @@
     color: #0b8043;
 }
 
-.security-summary-neutral .triangle-pointer,
-.security-summary-neutral .security-summary-text {
-    color: rgb(253, 177, 48);
-}
-
 .security-summary-insecure .triangle-pointer,
 .security-summary-insecure .security-summary-text,
 .security-explanation-title-neutral,