DLP: Make OSExchangeDataProvider::Clone copy source_

- Fixed OSExchangeDataProviderNonBacked::Clone
to copy the data source object.

Bug: 1154232
Change-Id: I2127ff9be7bbc5e9784b91644a8639ffea8b3893
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563722Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Aya Elsayed <ayaelattar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832540}

ui/base/dragdrop/os_exchange_data_provider_non_backed.cc

@@ -37,6 +37,9 @@ std::unique_ptr<OSExchangeDataProvider> OSExchangeDataProviderNonBacked::Clone()
   // We skip copying the drag images.
   clone->html_ = html_;
   clone->base_url_ = base_url_;
+  clone->source_ =
+      source_ ? std::make_unique<ui::DataTransferEndpoint>(*source_.get())
+              : nullptr;
 
   return clone;
 }