Out-of-line default constructor of WebRange.

WebRange default constructor is both inline and BLINK_EXPORT.
Using WebRange() in different blink modules (for example,
blink_core and blink_controller) results in duplicate symbols
during linking.

This CL out-of-lines this constructor to ensure that exported
constructor has only one implementation.

Bug: 903712
Change-Id: Ifdc947c4d0001a4be7404950a6a1639bc9877372
Reviewed-on: https://chromium-review.googlesource.com/c/1328964Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606878}

third_party/blink/public/web/web_range.h

@@ -45,7 +45,7 @@ class Range;
 class WebRange final {
  public:
   BLINK_EXPORT WebRange(int start, int length);
-  BLINK_EXPORT WebRange() {}
+  BLINK_EXPORT WebRange();
 
   int StartOffset() const { return start_; }
   int EndOffset() const { return end_; }

third_party/blink/renderer/core/exported/web_range.cc

@@ -46,6 +46,8 @@ WebRange::WebRange(int start, int length)
       << "These values are reserved to indicate that the range is null";
 }
 
+WebRange::WebRange() = default;
+
 WebRange::WebRange(const EphemeralRange& range) {
   if (range.IsNull())
     return;