Enable features API for content shell and webview

content/shell/browser/shell_browser_main_parts.cc

Index: content/shell/browser/shell_browser_main_parts.cc
diff --git a/content/shell/browser/shell_browser_main_parts.cc b/content/shell/browser/shell_browser_main_parts.cc
index 9b17cd58040a09ad74ba1528ea318e995e140ca1..ae965ddc6d96c8ad908bb48f5b52d2429400cb92 100644
--- a/content/shell/browser/shell_browser_main_parts.cc
+++ b/content/shell/browser/shell_browser_main_parts.cc
@@ -7,6 +7,7 @@
 #include "base/base_switches.h"
 #include "base/bind.h"
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/message_loop/message_loop.h"
@@ -133,6 +134,18 @@ void ShellBrowserMainParts::PreEarlyInitialization() {
 #endif
 }
 
+int ShellBrowserMainParts::PreCreateThreads() {
+    const base::CommandLine* command_line =

[Alexei Svitkine (slow), 2016/03/18 15:03:42]

Indent is wrong.

[Changwan Ryu, 2016/03/23 09:47:36]

Done.

+        base::CommandLine::ForCurrentProcess();
+    scoped_ptr<base::FeatureList> feature_list(new base::FeatureList);
+    feature_list->InitializeFromCommandLine(
+        command_line->GetSwitchValueASCII(switches::kEnableFeatures),
+        command_line->GetSwitchValueASCII(switches::kDisableFeatures));
+
+    base::FeatureList::SetInstance(std::move(feature_list));

[Alexei Svitkine (slow), 2016/03/18 15:03:42]

So this fails because an instance was already registered.

See this CL for how I initially made content shell initialize its FeatureList:

https://codereview.chromium.org/1409933007

The issue is that setting up the instance must only be done once. Right now,
this is done in Chrome code or in FeatureList::InitializeInstance() by content -
which is a no-op if Chrome code already initialized it.

You probably want to expand FeatureList::InitializeInstance() implementation -
with maybe passing the two flags to it.

[Changwan Ryu, 2016/03/23 09:47:36]

Please check crbug.com/596021 for the different approaches I took.
I could not find a perfect solution for this, but instead chose the one that
improves the situation. WDYT?

+    return 0;
+}
+
 void ShellBrowserMainParts::InitializeBrowserContexts() {
   set_browser_context(new ShellBrowserContext(false, net_log_.get()));
   set_off_the_record_browser_context(