Fix accidental launch of passthrough on OS_CHROMEOS

Because OS_LINUX includes OS_CHROMEOS, passthrough was accidentally
launched on ChromeOS by default. This was eventually refactored to
explicitly include OS_CHROMEOS, but we want to exclude that.

Bug: chromium:976283
Bug: chromium:1095700
Change-Id: I61f83056f91a174352cdddabce1d35b70ee3728a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419541Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Cr-Commit-Position: refs/heads/master@{#808631}

ui/gl/gl_features.cc

@@ -13,8 +13,7 @@ namespace features {
 // Launched on Windows, still experimental on other platforms.
 const base::Feature kDefaultPassthroughCommandDecoder{
   "DefaultPassthroughCommandDecoder",
-#if defined(OS_WIN) || ((defined(OS_LINUX) || defined(OS_CHROMEOS)) && \
-                        !defined(CHROMECAST_BUILD))
+#if defined(OS_WIN) || (defined(OS_LINUX) && !defined(OS_CHROMEOS))
       base::FEATURE_ENABLED_BY_DEFAULT
 #else
       base::FEATURE_DISABLED_BY_DEFAULT