DevTools: removing header 2 tag from EmptyWidget and BackgroundServiceView

Reason:
- EmptyWidget elements should not be headers as they do not meet the criteria (https://www.w3.org/TR/WCAG20-TECHS/G141.html)
- Marking a non-header as a header will impede SR accessibility for the affected tools

Changes:
- Replacing <h2> tag from UI.EmptyWidget with <div>
- Removing <h2> tag from _showPreview(...) function with <div> in BackgroundServiceView.js (supposed to mimic UI.EmptyWidget element)
- Adding 'empty-bold-text' css in emptyWidget.css that mimics <h2> styling and adding the class to the <div> tags to retain h2 styling

Examples of UI.EmptyWidget use cases:
https://imgur.com/a/qpJeB2Y

BackgroundServiceView.js unique case:
https://imgur.com/cAW8Nu0

Change-Id: Iff3356f12b3dd1906141de4cfe9980b7b1d9c929
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693808
Reviewed-by: Joel Einbinder <[email protected]>
Commit-Queue: Michael Liao <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#678026}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 58d8c299b23fad86a6fe7e2b489f9cdded704011
diff --git a/front_end/ui/emptyWidget.css b/front_end/ui/emptyWidget.css
index 3a25315..530bca8 100644
--- a/front_end/ui/emptyWidget.css
+++ b/front_end/ui/emptyWidget.css
@@ -4,6 +4,13 @@
  * found in the LICENSE file.
  */
 
+ .empty-bold-text {
+    display: block;
+    font-size: 1.5em;
+    margin: .83em 0 .83em;
+    font-weight: bold;
+ }
+
 .empty-view {
     color: hsla(0, 0%, 43%, 1);
     padding: 30px;