Add support for a fixed revision

In some cases a fixed revision of chromium is preferred, e.g. forbuilding docker images.
See scheib#5

README.md

@@ -2,3 +2,16 @@
 Scripts to download and run the latest Linux build of Chromium.
 Note that it is the latest development version, not the last stable one.
 This build is recommended for testing only.
+
+## Usage
+
+Download and runs the latest build of Chromium
+
+    $ ./update-and-run.sh
+
+Download a specific revision
+
+    $ export REVISION=492595
+    $ ./update-and-run.sh
+
+The same use is possible with ./update.sh

update.sh

@@ -4,9 +4,13 @@ cd $(dirname $0)
 
 LASTCHANGE_URL="https://www.googleapis.com/download/storage/v1/b/chromium-browser-snapshots/o/Linux_x64%2FLAST_CHANGE?alt=media"
 
-REVISION=$(curl -s -S $LASTCHANGE_URL)
+if [ -z "$REVISION" ]; then
+ REVISION=$(curl -s -S $LASTCHANGE_URL)
 
-echo "latest revision is $REVISION"
+  echo "latest revision is $REVISION"
+else
+  echo "Using given revision $REVISION as latest revision"
+fi
 
 if [ -d $REVISION ] ; then
   echo "already have latest version"