Prevent placeholder from unnecessarily wrapping (#2392)

Summary:
In Safari, the placeholder text sometimes wraps into multiple lines before it reaches the full width of the editor.
The addition of `width: 100%;` prevents the absolutely-positioned placeholder from trying to "shrink-wrap", which causes the unnecessarily wrapping.
Steps to reproduce:
1. Open the editor with a placeholder that takes up >50% but <100% of the editor width in Safari.
2. Type a character.
3. Press backspace.

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**Summary**

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**Test Plan**

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Pull Request resolved: #2392

Reviewed By: fryn

Differential Revision: D21319038

Pulled By: mrkev

fbshipit-source-id: b03c8afa47873ea210207aadb0225db8801bd84e

Author: fryn

src/component/base/DraftEditorPlaceholder.css

@@ -10,6 +10,7 @@
 .public/DraftEditorPlaceholder/root {
   color: var(fig-secondary-text);
   position: absolute;
+  width: 100%;
   z-index: 1;
 }