Uploading a new grcov release for use on mozilla central

There are three code coverage builds on mozilla-central we care about for grcov:

1. Linux 64-bit
2. Windows 64-bit (tests are run on 32-bit machines, so we have to upload a 32-bit grcov)
3. Mac 64-bit

The first step is to create a Bugzilla bug to track the update (unless there is already a bug on file).

Then, once we have built grcov for the three platforms (done by our CI automatically for Linux and Mac, we have to do it manually for now for Windows 32-bit until https://github.com/mozilla/grcov/issues/135 is fixed), we need to upload the three builds to tooltool following the guide at https://wiki.mozilla.org/ReleaseEngineering/Applications/Tooltool#How_To_Upload_To_Tooltool.

Linux example:

wget https://raw.githubusercontent.com/mozilla/build-tooltool/master/tooltool.py
python tooltool.py add --visibility public grcov-linux-x86_64.tar.bz2
python tooltool.py upload --authentication-file=~/.tooltool-token --message "Bug BUG_NUMBER - grcov $(git describe --abbrev=0 --tags) for Linux."

This will generate a manifest.tt file in the current working directory.

Now, we can update mozilla-central to use the new version of grcov, copying the "size" and "digest" values from manifest.tt to the ccov.manifests for a given platform.

The three manifests are at testing/config/tooltool-manifests/linux64/ccov.manifest, testing/config/tooltool-manifests/linux64/ccov.manifest, testing/config/tooltool-manifests/win32/ccov.manifest.