DevTools: shadow editor fields should not shrink

This adds styles to prevent fields in the CSS shadow editor from
shrinking. Bezier and Color swatch popovers are already scroll well
under small window dimensions.

Screenshot: https://imgur.com/a/hpIMe

Bug: 827093
Change-Id: I0ff7d4ca336cf728b79dac02feca4a0d60cff12f
Reviewed-on: https://chromium-review.googlesource.com/986661Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547885}

third_party/WebKit/Source/devtools/front_end/inline_editor/cssShadowEditor.css

@@ -9,10 +9,15 @@
     padding: 4px 12px 12px 12px;
 }
 
+.shadow-editor-field:last-of-type {
+    margin-bottom: 8px;
+}
+
 .shadow-editor-field {
     height: 24px;
     margin-top: 8px;
     font-size: 12px;
+    flex-shrink: 0;
 }
 
 .shadow-editor-flex-field {