Second try at #556. Uses absolute paths for the environment variables so that WPEDependencies builds properly. Switches from `WebKitBuildWPE` to `WebKitBuild/WPE` to avoid the need to change the .gitignore.
My computer takes a long time to build, but it appears to be working.
This patch:
- teaches `//browser_patches/webkit/build.sh` to accept the `--wpe` flag
- teaches `//browser_patches/webkit/archive.sh` to accept the `--wpe` flag
- teaches `//browser_patches/webkit/pw_run.sh` to parse the `--headless`
flag. In this case, we will assume that
`//browser_patches/webkit/checkout` is built for WPE and will pass
proper dependencies.