Fix textarea styling for firefox

` --iron-autogrow-textarea_-_white-space` needs to be
explicitly set to `pre-wrap` for textareas in firefox, without it,
the multi-line will render incorrectly.

This affects all `iron-autogrow-textarea`, so move it to
shared-styles as most of components are not using gr-textarea.

A todo might be moving to use `gr-textarea` in those components.

Bug: Issue 12551
Change-Id: Ie42d24ee88c5dc7ce9973d1dc5cf1e3164415580
(cherry picked from commit 927d8bb1da)

polygerrit-ui/app/elements/shared/gr-textarea/gr-textarea_html.js

@@ -50,9 +50,6 @@ export const htmlTemplate = html`
     }
     iron-autogrow-textarea {
       position: relative;
-
-      /** This is needed for firefox */
-      --iron-autogrow-textarea_-_white-space: pre-wrap;
     }
     #textarea.noBorder {
       border: none;

polygerrit-ui/app/styles/shared-styles.js

@@ -160,7 +160,12 @@ $_documentContainer.innerHTML = `<dom-module id="shared-styles">
           /* paper-tabs uses 0.8 here, but we want to control the color directly */
           opacity: 1;
           color: var(--deemphasized-text-color);
-        };      }
+        };
+      }
+      iron-autogrow-textarea {
+        /** This is needed for firefox */
+        --iron-autogrow-textarea_-_white-space: pre-wrap;
+      }
       strong {
         font-weight: var(--font-weight-bold);
       }