Just clone depot_tools in script/bootstrap instead of using a submodule

depot_tools likes to update itself, which makes using it as a submodule
annoying.

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "vendor/depot_tools"]
-	path = vendor/depot_tools
-	url = https://chromium.googlesource.com/chromium/tools/depot_tools.git
 [submodule "vendor/ninja"]
 	path = vendor/ninja
 	url = https://github.com/martine/ninja

script/bootstrap

@@ -5,10 +5,12 @@ set -e
 cd "$(dirname $0)/.."
 SOURCE_ROOT=$(pwd -P)
 VENDOR_DIR="${SOURCE_ROOT}/vendor"
+DEPOT_TOOLS_DIR="${VENDOR_DIR}/depot_tools"
 NINJA_DIR="${VENDOR_DIR}/ninja"
 
 git submodule sync
 git submodule update --init --recursive
+test -d "${DEPOT_TOOLS_DIR}" || git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git "${DEPOT_TOOLS_DIR}"
 
 "${NINJA_DIR}/bootstrap.py"
 

vendor/.gitignore

@@ -0,0 +1 @@
+/depot_tools/

vendor/depot_tools

@@ -1 +0,0 @@
-Subproject commit 34b5d8268644a40faea2d01d9b57d2ace49d6a22