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"
+
+# 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
+
+# Remove all except white-listed.
+cd $TARGET_DIR
+find . -type f | grep -Fxvf ../WPTWhiteList | xargs -n 1 rm
+find . -empty -type d -delete
+
+# TODO(burnik): Handle the SSL certs and other configuration.