Change 'lookUpwardForInlineStyle' from O(n^2) to O(n).

Summary:
**Summary**

The function 'lookUpwardForInlineStyle' repeatedly called 'getPreviousBlock' which has the complexity O(n). This tanks performance when creating a document with lots of empty lines. Simply holding down the enter key will quickly cause the frame rate to drop below 1fps.

The fix is straightforward and, in my opinion, preferable to a while loop.

**Test Plan**

I did some manual testing. The code was able to correctly (and efficiently) find the correct style.
Closes facebookarchive/draft-js#1429

Differential Revision: D6060417

fbshipit-source-id: 8a14af25f776ac5263747e2fc4bbb665884ad970

src/model/immutable/EditorState.js

@@ -652,17 +652,15 @@ function lookUpwardForInlineStyle(
   content: ContentState,
   fromKey: string,
 ): DraftInlineStyle {
-  var previousBlock = content.getBlockBefore(fromKey);
-  var previousLength;
-
-  while (previousBlock) {
-    previousLength = previousBlock.getLength();
-    if (previousLength) {
-      return previousBlock.getInlineStyleAt(previousLength - 1);
-    }
-    previousBlock = content.getBlockBefore(previousBlock.getKey());
-  }
-
+  var lastNonEmpty = content.getBlockMap()
+    .reverse()
+    .skipUntil((_, k) => k === fromKey)
+    .skip(1)
+    .skipUntil((block, _) => block.getLength())
+    .first();
+
+  if (lastNonEmpty)
+    return lastNonEmpty.getInlineStyleAt(lastNonEmpty.getLength() - 1);
   return OrderedSet();
 }