devops: fetch chromium checkout if it has not been before (#5169)

Checkouts should reside outside of gihtub action working directory
so will be reused between builds.

browser_patches/chromium/build.sh

@@ -95,6 +95,18 @@ compile_chromium() {
   # Update Chromium checkout. One might think that this step should go to `prepare_checkout.sh`
   # script, but the `prepare_checkout.sh` is in fact designed to prepare a fork checkout, whereas
   # we don't fork Chromium.
+  #
+  # This is based on https://chromium.googlesource.com/chromium/src/+/master/docs/linux/build_instructions.md#get-the-code
+  if [[ ! -d "${CR_CHECKOUT_PATH}" ]]; then
+    mkdir "${CR_CHECKOUT_PATH}"
+    cd "${CR_CHECKOUT_PATH}"
+    fetch --no-hooks chromium
+    cd src
+    if [[ $(uname) == "Linux" ]]; then
+      ./build/install-build-deps.sh
+    fi
+    gclient runhooks
+  fi
   cd "${CR_CHECKOUT_PATH}/src"
   git checkout master
   git pull origin master