Update read later menu title size based on UX feedback.

Bug: 1109316
Change-Id: I797c98ff77d57ab47dfb95eecfe6d70000f3409d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500561
Auto-Submit: Caroline Rising <corising@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Reviewed-by: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821282}

chrome/browser/resources/read_later/app.html

@@ -2,7 +2,7 @@
   #header {
     align-items: center;
     display: flex;
-    font-size: var(--mwb-primary-text-font-size);
+    font-size: 15px;
     height: var(--mwb-item-height);
     margin: 0;
     padding-inline-start: var(--mwb-list-item-horizontal-margin);