Tab Search: List item content visibility optimization

Leverage content-visibility property to optimize render time.
https://web.dev/content-visibility/

Screenshots from Dev Tools: Performance:
original: https://drive.google.com/file/d/1bb_KeUWn1C0TDmUuhZO2zE0qcqqDlq3O/view?usp=sharing
optimized: https://drive.google.com/file/d/1yHm19aq3ri7YnI9xUpPbfmVQCycuESXF/view?usp=sharing

6x-slow screenshots:
original: https://drive.google.com/file/d/13ymyxUmpPWHNh0z-I_s_wUN2osGFwJW_/view?usp=sharing
optimized: https://drive.google.com/file/d/1y7MPjiU9px2S0BlfRlCizKG9b9kj9tHi/view?usp=sharing

Bug: 1099917
Change-Id: I2206817422cbaeb4a59ed1707a2e39dd354cc7ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493092Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820498}

ui/webui/resources/cr_elements/mwb_shared_style.html

@@ -32,6 +32,8 @@
     .mwb-list-item {
       align-items: center;
       background-color: var(--mwb-background-color);
+      contain-intrinsic-size: var(--mwb-item-height);
+      content-visibility: auto;
       display: flex;
       height: var(--mwb-item-height);
       padding: 0 var(--mwb-list-item-horizontal-margin);