Add missing explicit modifier to FindBuffer constructor

This patch adds missing |explicit| modifier to |FindBuffer| constructor to avoid
implicit type conversion for improving code health.


This patch is follow-up of CL[1] which introduces this constructor.

[1] http://crrev.com/c/1424677 Allow FindBuffer to find text bounded by a range

Change-Id: Ic4bf4d675ef26ff36687a6cfb1feb995e20a37de
Reviewed-on: https://chromium-review.googlesource.com/c/1454056Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629044}

third_party/blink/renderer/core/editing/finder/find_buffer.h

@@ -22,7 +22,7 @@ class CORE_EXPORT FindBuffer {
   STACK_ALLOCATED();
 
  public:
-  FindBuffer(const EphemeralRangeInFlatTree& range);
+  explicit FindBuffer(const EphemeralRangeInFlatTree& range);
 
   static EphemeralRangeInFlatTree FindMatchInRange(
       const EphemeralRangeInFlatTree& range,