Introduce chrome.displaySource API

extensions/common/permissions/extensions_api_permissions.cc

Index: extensions/common/permissions/extensions_api_permissions.cc
diff --git a/extensions/common/permissions/extensions_api_permissions.cc b/extensions/common/permissions/extensions_api_permissions.cc
index c9eb11b5e7944e4f9ac16f03ac1f6f5d7f416adf..40e2db9126e1b7ddfae2bb86fbe57c84e3b96bf2 100644
--- a/extensions/common/permissions/extensions_api_permissions.cc
+++ b/extensions/common/permissions/extensions_api_permissions.cc
@@ -54,6 +54,8 @@ std::vector<APIPermissionInfo*> ExtensionsAPIPermissions::GetAllPermissions()
       {APIPermission::kDiagnostics,
        "diagnostics",
        APIPermissionInfo::kFlagCannotBeOptional},
+      {APIPermission::kDisplaySource, "displaySource",
+       APIPermissionInfo::kFlagCannotBeOptional},

[asargent_no_longer_on_chrome, 2015/11/17 00:51:56]

Is there a reason this can't this be an optional permission, or did you just
copy the kFlagCannotBeOptional flag because other permissions here do the same
thing? 

For more background if you're unfamiliar with optional permissions, please see
https://developer.chrome.com/extensions/permissions

As a general rule all APIs should support optional permissions except for a very
few edge cases.  

[Mikhail, 2015/11/17 14:07:42]

Here I took after cast API which has quite similar functionality. The
kFlagCannotBeOptional is removed now.

       {APIPermission::kDns, "dns"},
       {APIPermission::kDocumentScan, "documentScan"},
       {APIPermission::kExtensionView,