devops: revision Chromium repackaged builds separately (#3698)

Currently, we mirror Chromium revisions from gbuckets to our cdn and
name them same way as Chromium revision.

However, with the upcoming bundling of FFMPEG, we'd like to revision
Chromium packages separately, since our Chromium package will depend from
a number of factors:
- chromium upstream revision
- ffmpeg version
- extra files to add to the package or remove from the package

We should be able to produce a new Chromium build once any of these changes.

With this patch, to roll Chromium browser:
- update chromium revision number in the
`//browser_patches/chromium/UPSTREAM_CONFIG.sh`
- bump the build number in the `//browser_patches/chromium/BUILD_NUMBER`

Reference #3680

browser_patches/chromium/BUILD_NUMBER

@@ -1 +1 @@
-799610
+1000

browser_patches/chromium/UPSTREAM_CONFIG.sh

@@ -0,0 +1 @@
+UPSTREAM_CHROMIUM_REVISION=792639

browser_patches/chromium/build.sh

@@ -5,10 +5,11 @@ set +x
 trap "cd $(pwd -P)" EXIT
 cd "$(dirname $0)"
 
+source "./UPSTREAM_CONFIG.sh"
+
 mkdir -p output
 cd output
 
-BUILD_NUMBER=$(head -1 ../BUILD_NUMBER)
 FOLDER_NAME=""
 ZIP_NAME=""
 FILES_TO_REMOVE=()
@@ -32,7 +33,7 @@ else
   exit 1
 fi
 
-URL="https://storage.googleapis.com/chromium-browser-snapshots/${FOLDER_NAME}/${BUILD_NUMBER}/${ZIP_NAME}"
+URL="https://storage.googleapis.com/chromium-browser-snapshots/${FOLDER_NAME}/${UPSTREAM_CHROMIUM_REVISION}/${ZIP_NAME}"
 curl --output upstream.zip "${URL}"
 
 unzip upstream.zip