GCC: remove noexcept on definition of operator= in PaintPreviewClient

GCC throws an error if noexcept is specified on declaration and
definition. Specifing on declaration is enough.

Bug: 819294
Change-Id: I89f03cf3ee73e63b3f711082315a67f93ccc9dbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2059999Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Mehran Mahmoudi <mahmoudi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742632}

components/paint_preview/browser/paint_preview_client.cc

@@ -98,7 +98,7 @@ PaintPreviewClient::PaintPreviewData::PaintPreviewData() = default;
 PaintPreviewClient::PaintPreviewData::~PaintPreviewData() = default;
 
 PaintPreviewClient::PaintPreviewData& PaintPreviewClient::PaintPreviewData::
-operator=(PaintPreviewData&& rhs) noexcept = default;
+operator=(PaintPreviewData&& rhs) = default;
 
 PaintPreviewClient::PaintPreviewData::PaintPreviewData(
     PaintPreviewData&& other) noexcept = default;