Add libcurl headers and examples to //components/cronet/third_party.

This doesn't include the libcurl implementation -- just headers and
samples to build an alternative implementation of the libcurl API.

The full libcurl repo with its implementation and repo history would be
nice to have, but not strictly necessary.

Since this code isn't intended to be shipped with Chrome, but rather as
headers for an alternative API for the components/cronet library, it
makes sense to avoid having Chromium developers sync the full library.

Bug: 973603
Change-Id: I3879241ab7b2aa0177cd09cc2e8ef73739fd5feb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1664790
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672184}
135 files changed