Add missing |noexcept| keyword to our move-constructors.

Noticed while woring on follow-up cl at
  https://pdfium-review.googlesource.com/c/pdfium/+/73510

Change-Id: Ic2a3d780a8692378107ada25dc6e428bf28153e8
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/73511
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcrt/widestring.cpp b/core/fxcrt/widestring.cpp
index b51ed68..813f2b8 100644
--- a/core/fxcrt/widestring.cpp
+++ b/core/fxcrt/widestring.cpp
@@ -410,7 +410,7 @@
   return *this;
 }
 
-WideString& WideString::operator=(WideString&& that) {
+WideString& WideString::operator=(WideString&& that) noexcept {
   if (m_pData != that.m_pData)
     m_pData = std::move(that.m_pData);