WebUI: fix cr-input label font weight

Improves a11y (readability).

Bug: 870937
Change-Id: Id07c24f7661af0139595b59b056cb5d5a2481894
Reviewed-on: https://chromium-review.googlesource.com/1164244Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581089}

ui/webui/resources/cr_elements/shared_vars_css.html

@@ -146,6 +146,7 @@
       color: var(--google-grey-refresh-700);
       display: block;
       font-size: var(--cr-form-field-label-font-size);
+      font-weight: 500;
       letter-spacing: 0.4px;
       line-height: var(--cr-form-field-label-font-size);
       margin-bottom: 8px;