[HeadersView] Enforce using lower case header names internally

Drive-by: header value is of type string|null

Bug: 1297533
Change-Id: I6f59658ba846a5da223037da66ea704fe09ae4db
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3870495
Commit-Queue: Wolfgang Beyer <[email protected]>
Reviewed-by: Simon Zünd <[email protected]>
diff --git a/front_end/panels/network/components/RequestHeadersView.ts b/front_end/panels/network/components/RequestHeadersView.ts
index 88431db..fc80357 100644
--- a/front_end/panels/network/components/RequestHeadersView.ts
+++ b/front_end/panels/network/components/RequestHeadersView.ts
@@ -425,7 +425,7 @@
 
   #renderGeneralRow(name: Common.UIString.LocalizedString, value: string, classNames?: string[]): LitHtml.LitTemplate {
     const isHighlighted = this.#toReveal?.section === NetworkForward.UIRequestLocation.UIHeaderSection.General &&
-        name.toUpperCase() === this.#toReveal?.header?.toUpperCase();
+        name.toLowerCase() === this.#toReveal?.header?.toLowerCase();
     return html`
       <div class="row ${isHighlighted ? 'header-highlight' : ''}">
         <div class="header-name">${name}:</div>