Disable widevine on Linux

media: Add assert for "enable_widevine" gn arg
https://chromium-review.googlesource.com/957707

chromiumcontent/args/ffmpeg.gn

@@ -3,11 +3,15 @@ is_component_build = false
 is_mas_build = getenv("MAS_BUILD") != ""
 is_debug = false
 enable_nacl = false
-enable_widevine = true
 proprietary_codecs = false
 is_component_ffmpeg = true
 ffmpeg_branding = "Chromium"
 
+# As of Chromium 67.0.3396.99 widevine is only supported on Mac and Windows.
+if (target_os != "linux") {
+  enable_widevine = true
+}
+
 # Use the system provided standard library on platforms other than Linux.
 if (target_os != "linux") {
   use_custom_libcxx = false

chromiumcontent/args/shared_library.gn

@@ -6,7 +6,6 @@ is_debug = true
 symbol_level = 2
 use_allocator = "none"    # on Linux this defaults to "tcmalloc", but tcmalloc deadlocks when Electron CI tests are run inside Docker
 enable_nacl = false
-enable_widevine = true
 enable_cdm_host_verification = false
 proprietary_codecs = true
 is_component_ffmpeg = true
@@ -19,6 +18,11 @@ ffmpeg_branding = "Chrome"
 # been built with H.264 support.
 rtc_use_h264 = true
 
+# As of Chromium 67.0.3396.99 widevine is only supported on Mac and Windows.
+if (target_os != "linux") {
+  enable_widevine = true
+}
+
 # Use the system provided standard library on platforms other than Linux.
 if (target_os != "linux") {
   use_custom_libcxx = false

chromiumcontent/args/static_library.gn

@@ -5,7 +5,6 @@ is_mas_build = getenv("MAS_BUILD") != ""
 is_official_build = true
 symbol_level = 2
 enable_nacl = false
-enable_widevine = true
 enable_cdm_host_verification = false
 proprietary_codecs = true
 is_component_ffmpeg = true
@@ -18,6 +17,11 @@ ffmpeg_branding = "Chrome"
 # been built with H.264 support.
 rtc_use_h264 = true
 
+# As of Chromium 67.0.3396.99 widevine is only supported on Mac and Windows.
+if (target_os != "linux") {
+  enable_widevine = true
+}
+
 # CFI is disabled for the time being, as Electron is not a monolithic binary
 # with at least one shared library component (Node) and CFI is tricky in that
 # scenario