Changes imported from Abseil "staging" branch:

- 5b675ef65e4977b3ac778a75a21e99db1ebe78e7 Remove "not an official google project" disclaimer. by Abseil Team <absl-team@google.com>
  - 77d2aacc03efe6841612b38bcbb745dde1ad7d3e Avoid weak virtual table warnings (-Wweak-vtables) and re... by Abseil Team <absl-team@google.com>

GitOrigin-RevId: 5b675ef65e4977b3ac778a75a21e99db1ebe78e7
Change-Id: Ia0d1d6e39169c7ad9783d25dc92dad041de3a966

README.md

@@ -84,7 +84,3 @@ For more information about Abseil:
 * Peruse our
   [Abseil Compatibility Guarantees](http://abseil.io/about/compatibility) to
   understand both what we promise to you, and what we expect of you in return.
-
-## Disclaimer
-
-*   This is not an official Google product.

absl/base/internal/malloc_extension.cc

@@ -29,6 +29,13 @@ namespace base_internal {
 SysAllocator::~SysAllocator() {}
 void SysAllocator::GetStats(char* buffer, int) { buffer[0] = 0; }
 
+// Dummy key method to avoid weak vtable.
+void MallocExtensionWriter::UnusedKeyMethod() {}
+
+void StringMallocExtensionWriter::Write(const char* buf, int len) {
+  out_->append(buf, len);
+}
+
 // Default implementation -- does nothing
 MallocExtension::~MallocExtension() { }
 bool MallocExtension::VerifyAllMemory() { return true; }

absl/base/internal/malloc_extension.h

@@ -388,6 +388,9 @@ class MallocExtensionWriter {
   MallocExtensionWriter() {}
   MallocExtensionWriter(const MallocExtensionWriter&) = delete;
   MallocExtensionWriter& operator=(const MallocExtensionWriter&) = delete;
+
+ private:
+  virtual void UnusedKeyMethod();  // Dummy key method to avoid weak vtable.
 };
 
 // A subclass that writes to the std::string "out".  NOTE: The generated
@@ -396,9 +399,7 @@ class MallocExtensionWriter {
 class StringMallocExtensionWriter : public MallocExtensionWriter {
  public:
   explicit StringMallocExtensionWriter(std::string* out) : out_(out) {}
-  virtual void Write(const char* buf, int len) {
+  void Write(const char* buf, int len) override;
-    out_->append(buf, len);
-  }
 
  private:
   std::string* const out_;