Update web-platform-tests.
embedding: add some commonly used string constants from chromium
embedding: implement cef_execute_process() stub

this is missing signal handler functionality in rust and we currently have no use case for actual execution of secondary processes
embedding: cef_string_wide_cmp()
embedding: cef_string_utf8_to_wide()
embedding: cef_string_wide_to_utf8()
embedding: cef_string_ascii_to_utf16()
embedding: cef_string_ascii_to_wide()
auto merge of #4044 : zmike/servo/embedding-process, r=jdm

Fixes #4023 

@SimonSapin @jdm