feat: introduce puppeteer-firefox (#3628)

This adds a proof-of-concept of `puppeteer-firefox`.
This consists of two parts:
- `//experimental/juggler` - patches to apply to Firefox.
- `//experimental/puppeteer-firefox` - front-end code to
be merged with Puppeteer.

As things become more stable, we'll gradually move it out of
the experimental folder.

experimental/juggler/scripts/fetch_firefox.sh

@@ -0,0 +1,18 @@
+set -e
+set -x
+
+if [ -d $SOURCE/firefox ]; then
+  echo ERROR! Directory "${SOURCE}/firefox" exists. Remove it and re-run this script.
+  exit 1;
+fi
+mkdir -p $SOURCE/firefox
+cd $SOURCE/firefox
+git init
+git remote add origin https://github.com/mozilla/gecko-dev.git
+git fetch --depth 50 origin release
+git reset --hard $(cat $SOURCE/FIREFOX_SHA)
+if [[ $? == 0 ]]; then
+  echo SUCCESS
+else
+  echo FAILED TO CHECKOUT PINNED REVISION
+fi

experimental/juggler/scripts/install_gcloud.sh

@@ -0,0 +1,9 @@
+# auth
+echo $GS_AUTH > $SOURCE/gsauth
+# install gcloud sdk
+curl https://dl.google.com/dl/cloudsdk/release/google-cloud-sdk.tar.gz > /tmp/google-cloud-sdk.tar.gz
+mkdir -p $SOURCE/gcloud \
+  && tar -C $SOURCE/gcloud -xvf /tmp/google-cloud-sdk.tar.gz \
+  && CLOUDSDK_CORE_DISABLE_PROMPTS=1 $SOURCE/gcloud/google-cloud-sdk/install.sh
+gcloud auth activate-service-account --key-file=$SOURCE/gsauth
+gcloud config set project juggler-builds

experimental/juggler/scripts/upload_linux.sh

@@ -0,0 +1,13 @@
+set -e
+
+if [ -e ./FIREFOX_SHA ]; then
+  echo Checking Juggler root - OK
+else
+  echo Please run this script from the Juggler root
+  exit 1;
+fi
+cd firefox/obj-x86_64-pc-linux-gnu/dist/
+zip -r firefox-linux.zip firefox
+mv firefox-linux.zip ../../../
+cd -
+gsutil mv firefox-linux.zip gs://juggler-builds/$(git rev-parse HEAD)/

experimental/juggler/scripts/upload_mac.sh

@@ -0,0 +1,13 @@
+set -e
+
+if [ -e ./FIREFOX_SHA ]; then
+  echo Checking Juggler root - OK
+else
+  echo Please run this script from the Juggler root
+  exit 1;
+fi
+cd firefox/obj-x86_64-apple-darwin17.7.0/dist/
+zip -r firefox-mac.zip firefox
+mv firefox-mac.zip ../../../
+cd -
+gsutil mv firefox-mac.zip gs://juggler-builds/$(git rev-parse HEAD)/