Remove the enable_task_manager build flag and define.

These have been replaced by !android checks. The task manager is enabled on all
platforms except Android and iOS, and code in chrome isn't compiled on iOS.

BUG=671706

Review-Url: https://codereview.chromium.org/2558143002
Cr-Original-Commit-Position: refs/heads/master@{#437281}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ee486bb1c7a7351039d88ba233ad9eabc94887df

config/BUILD.gn

@@ -136,9 +136,6 @@ config("feature_flags") {
   if (!enable_nacl) {
     defines += [ "DISABLE_NACL" ]
   }
-  if (enable_task_manager) {
-    defines += [ "ENABLE_TASK_MANAGER=1" ]
-  }
   if (enable_themes) {
     defines += [ "ENABLE_THEMES=1" ]
   }

config/features.gni

@@ -79,8 +79,6 @@ declare_args() {
 
 # Additional dependent variables -----------------------------------------------
 
-enable_task_manager = !is_ios && !is_android
-
 enable_themes = !is_android && !is_ios
 
 # Whether we are using the rlz library or not.  Platforms like Android send