DevTools [Network Conditions]: adding error text for network condition tool custom user agent text input

Re-upload of https://chromium-review.googlesource.com/c/chromium/src/+/1730131 to DevTools repository

Changes:
- added error text div and needed css
- adding value length checks on creation and input

**Note: the text now reads "Custom user agent field is required"
Image of error text:
https://imgur.com/8PI75T3

GIF of error text behavior:
https://imgur.com/o8oZvcW

New error element spacing:
https://imgur.com/hE3Xmli

Bug: 963183
Change-Id: Ib5776ae72d4abaabf03efce5838ddb731283c0ab
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1871820
Reviewed-by: Lorne Mitchell <[email protected]>
Commit-Queue: Michael Liao <[email protected]>
diff --git a/front_end/network/networkConfigView.css b/front_end/network/networkConfigView.css
index 6cb6445..3dfc096 100644
--- a/front_end/network/networkConfigView.css
+++ b/front_end/network/networkConfigView.css
@@ -40,6 +40,11 @@
     padding-top: 0;
 }
 
+.network-config-input-validation-error {
+  color: var(--input-validation-error);
+  margin: 5px 0;
+}
+
 /* Network throttling */
 
 .network-config-throttling .chrome-select {