chromium: Update all channels to latest versions.

This closes #1623, and updates _all_ channels to the corresponding
latest upstream versions.

Thanks to @wizeman for opening the pull request noted above and for
another update in between, @aristidb for fixing the patcheShebangs issue
and @shlevy for notifying me about the build failure in stdenv-updates
in the first place.

Sorry to everyone for my inactivity lately.

The following changes were needed in order to build those new releases:

 * Patch out /bin/echo to allow building with all options enabled.
 * Always use GN from the source tree.
 * Remove import of depot_tools for version 34.
 * Drop version 32 specific stuff.

With this commit, the following new upstream versions are introduced:

stable: 32.0.1700.77 -> 32.0.1700.102 (builds fine, tested)
beta:   32.0.1700.19 -> 33.0.1750.46  (builds fine, tested)
dev:    33.0.1712.4  -> 34.0.1809.0   (build broken with gnome_keyring)

The dev version requires a more recent version gnome_keyring and thus
won't build if gnomeKeyringSupport is set to true. I haven't tested this
build without gnomeKeyringSupport yet, so it might be broken and will be
fixed later.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>