Use github link to SVG example image in docs.

It seems like there isn't a way to reference a local SVG in gitiles.
We'd need to upload the image to cloud storage. Instead we can use
the image in the repo if we link to the github version.

Bug: angleproject:3002
Change-Id: Ib1879d562e83ae16f531b204ca86cb73d45c5bb8
Reviewed-on: https://chromium-review.googlesource.com/c/1459600
Reviewed-by: Jamie Madill <jmadill@chromium.org>

doc/DirtyBits.md

@@ -74,7 +74,7 @@ Framebuffer- ->Context: onSubjectStateChange
 Note over Context: cache update
 -->
 
-![State Change Example](https://chromium.googlesource.com/angle/angle/+/master/doc/img/StateNotificationExample.svg)
+![State Change Example](https://raw.githubusercontent.com/google/angle/master/doc/img/StateNotificationExample.svg?sanitize=true)
 
 We use the [Observer pattern](https://en.wikipedia.org/wiki/Observer_pattern) to implement cache
 invalidation notifications. See [`Observer.h`](../src/libANGLE/Observer.h). In the example the