Use std::exchange() idiom to implement SharedMemoryMapping move ctor.

Bug: none
Change-Id: I0dda0908227f65cda8a35931e83fd94a308c6f91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083563
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746486}

base/memory/shared_memory_mapping.cc

@@ -34,21 +34,18 @@ namespace base {
 SharedMemoryMapping::SharedMemoryMapping() = default;
 
 SharedMemoryMapping::SharedMemoryMapping(SharedMemoryMapping&& mapping) noexcept
-    : memory_(mapping.memory_),
+    : memory_(std::exchange(mapping.memory_, nullptr)),
       size_(mapping.size_),
       mapped_size_(mapping.mapped_size_),
-      guid_(mapping.guid_) {
-  mapping.memory_ = nullptr;
-}
+      guid_(mapping.guid_) {}
 
 SharedMemoryMapping& SharedMemoryMapping::operator=(
     SharedMemoryMapping&& mapping) noexcept {
   Unmap();
-  memory_ = mapping.memory_;
+  memory_ = std::exchange(mapping.memory_, nullptr);
   size_ = mapping.size_;
   mapped_size_ = mapping.mapped_size_;
   guid_ = mapping.guid_;
-  mapping.memory_ = nullptr;
   return *this;
 }