Update CHANGELOG for tcmalloc change

CHANGELOG.md

 ## 1.20.0
 
 ### Dart VM
 
 * We have improved the way that the VM locates the native code library for a
   native extension (e.g. `dart-ext:` import). We have updated this
   [article on native extensions](https://www.dartlang.org/articles/dart-vm/native-extensions)
   to reflect the VM's improved behavior.
 
+* Linux builds of the VM will now use the tcmalloc library for memory
+  allocation. This has the advantages of better debugging and profiling support
+  and faster small allocations, with the cost of slightly larger initial memory
+  footprint, and slightly slower large allocations.
+
 * We have improved the way the VM searches for trusted root certificates for
   secure socket connections on Linux. First, the VM will look for trusted root
   certificates in standard locations on the file system
   (/etc/pki/tls/certs/ca-bundle.crt followed by /etc/ssl/certs), and only if
   these do not exist will it fall back on the builtin trusted root certificates.
   This behavior can be overridden on Linux with the new flags
   --root-certs-file and --root-certs-cache. The former is the path to a file
   containing the trusted root certificates, and the latter is the path to a
   directory containing root certificate files hashed using `c_rehash`.
 
@@ skipping 1246 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.