Introduce WPTServe for running W3C Blink Layout tests

Tools/Scripts/webkitpy/thirdparty/wpt/checkout.sh

+#!/bin/bash
+#
+# Removes ./wpt/ directory containing the reduced web-platform-tests tree and
+# starts a new checkout. Only files in WPTWhiteList are retained. The revisions
+# getting checked out are defined in WPTHeads.
+
+DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
+cd $DIR
+
+TARGET_DIR=$DIR/wpt
+REMOTE_REPO="https://chromium.googlesource.com/external/w3c/web-platform-tests.git"
+
+function clone {
+  # First line is the main repo HEAD.
+  WPT_HEAD=$(head -n 1 $DIR/WPTHeads)
+
+  # Remove existing repo if already exists.
+  [ -d "$TARGET_DIR" ] && rm -rf $TARGET_DIR
+
+  # Clone the main repository.
+  git clone $REMOTE_REPO $TARGET_DIR
+  cd $TARGET_DIR && git checkout $WPT_HEAD
+
+  # Starting from the 2nd line of WPTWhiteList, we read and checkout submodules.
+  tail -n+2 $DIR/WPTHeads | while read dir submodule commit; do
+    cd $TARGET_DIR/$dir && \
+      git submodule update --init $submodule && \
+      cd $TARGET_DIR/$dir/$submodule && \
+      git checkout $commit
+  done
+}
+
+function reduce {
+  # Remove all except white-listed.
+  cd $TARGET_DIR
+  find . -type f | grep -Fxvf ../WPTWhiteList | xargs -n 1 rm
+  find . -empty -type d -delete
+}
+
+actions="clone reduce"
+[ "$1" != "" ] && actions="$@"
+
+for action in $actions; do
+  type -t $action >/dev/null || (echo "Unknown action: $action" 1>&2 && exit 1)
+  $action
+done
+
+# TODO(burnik): Handle the SSL certs and other configuration.