cros: Make GetChannelName() call GetChannel().

This is a late follow-up to
https://chromium-review.googlesource.com/c/chromium/src/+/1287850 --
since GetChannelName() now depends on GetChannel() being called
previously, call it instead of relying on something else having
called it.

Probably a no-op in practice, but it makes the code more reliable
and look less surprising.

Bug: none
Change-Id: Ie4e2c791db52f590d44d906fb5b0a42f521d427e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887270
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710412}

chrome/common/channel_info_chromeos.cc

@@ -33,7 +33,7 @@ void SetChannel(const std::string& channel) {
 
 std::string GetChannelName() {
 #if BUILDFLAG(GOOGLE_CHROME_BRANDING)
-  switch (g_chromeos_channel) {
+  switch (GetChannel()) {
     case version_info::Channel::STABLE:
       return std::string();
     case version_info::Channel::BETA: