Specify line-height for code in terms of rem

We currently see 1px horizontal lines that originate from some <tr>s
being 17px instead of 16px. Apparently changing line-height to use `rem`
instead of a multiplier fixes the issue.

Change-Id: I4eec06f6fb02cf344e4ced1a6087810b3fa6512a
diff --git a/polygerrit-ui/app/styles/themes/app-theme.ts b/polygerrit-ui/app/styles/themes/app-theme.ts
index 9b62718..61e1a7b 100644
--- a/polygerrit-ui/app/styles/themes/app-theme.ts
+++ b/polygerrit-ui/app/styles/themes/app-theme.ts
@@ -122,7 +122,7 @@
     --font-size-h3: 1.143rem;   /* 16px */
     --font-size-h2: 1.429rem;   /* 20px */
     --font-size-h1: 1.714rem;   /* 24px */
-    --line-height-code: 1.334;      /* 16px */
+    --line-height-code: 1.143rem;   /* 16px */
     --line-height-mono: 1.286rem;   /* 18px */
     --line-height-small: 1.143rem;  /* 16px */
     --line-height-normal: 1.429rem; /* 20px */