update changelog for Dartium roll

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c38a97d15ff2844031df7eff6f7b57d772f83f16..99d8a0083a218b49dca8d8417861d60e4b6fcab3 100644
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+++ b/CHANGELOG.md
@@ -23,6 +23,17 @@
     `SecurityContext.usePrivateKeyBytes`, for use as the password for PKCS12
     data.
 
+### Dartium
+
+  * The Chrome-based tools that ship as part of the Dart SDK – Dartium and
+    content shell – are now based on Chrome version 45 (instead of Chrome 39).
+  * Dart browser libraries (`dart:html`, `dart:svg`, etc) have not been updated.
+    * These are still based on Chrome 39.
+    * These APIs will be updated in a future release.
+  * Note that there are experimental APIs which have changed in the underlying
+    browser, and will not work with the older libraries.
+    For example, `Element.animate`.
+
 ### Service protocol changes
 
 * Fixed a documentation bug where the field `extensionRPCs` in `Isolate`