Create the base Custom Context Menu Dialog.

This CL creates the new Context Menu. There are a few key decisions I
made during each moment and if the explanation isn't in here, please
take a look at the design doc under caveats or just talk to me! (Usually
we can get things cleared up a little quicker through in person chat
than through conversations).

This change covers the base dialog and the feature flag for it. Please
take a look when you can. Image loading, Share Helper, and contextmenu
-> context_menu will all come in different cls.

----Screenshots----
https://drive.google.com/open?id=0B6D5A57VLDpeSHZfNkgzUlVhYlk
https://drive.google.com/open?id=0B6D5A57VLDpeVDNHUXFOVzVnbFU

Video - https://drive.google.com/open?id=0B6D5A57VLDpeaUNVNXFReVNnYUk

DESIGN_DOC=
https://docs.google.com/document/d/1MvLS1sRJ6CqOdSg8sKzA0j2gbKO2XGRAb02SPlK1cbw/edit#heading=h.xgjl2srtytjt

BUG=613357

Review-Url: https://codereview.chromium.org/2751333006
Cr-Commit-Position: refs/heads/master@{#460664}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index c33cfb5b..e3d084e 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2485,6 +2485,13 @@
                                     kDelayNavigationFeatureVariations,
                                     "DelayNavigation")},
 
+#if defined(OS_ANDROID)
+    {"enable-custom-context-menu",
+     flag_descriptions::kEnableCustomContextMenuName,
+     flag_descriptions::kEnableCustomContextMenuDescription, kOsAndroid,
+     FEATURE_VALUE_TYPE(chrome::android::kCustomContextMenu)},
+#endif  // OS_ANDROID
+
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in
     // histograms.xml and don't forget to run AboutFlagsHistogramTest unit test.