Reland 83186: Enable building base.dll for the multi-dll build.

base/base.gypi

Index: base/base.gypi
===================================================================
--- base/base.gypi	(revision 83366)
+++ base/base.gypi	(working copy)
@@ -423,7 +423,7 @@
   'targets': [
     {
       'target_name': 'base',
-      'type': '<(library)',
+      'type': '<(component)',
       'msvs_guid': '1832A374-8A74-4F9E-B536-69A699B3E165',
       'variables': {
         'base_target': 1,
@@ -528,6 +528,26 @@
               'win_util.cc',
             ],
         },],
+        [ 'OS=="win" and component=="shared_library"', {
+          'defines': [
+            'BASE_DLL',
+            'BASE_IMPLEMENTATION=1',
+          ],
+          'msvs_disabled_warnings': [
+            4251,
+          ],
+          'sources!': [
+            'debug/debug_on_start_win.cc',
+          ],
+          'direct_dependent_settings': {
+            'defines': [
+              'BASE_DLL',
+            ],
+            'msvs_disabled_warnings': [
+              4251,
+            ],
+          },
+        }],
       ],
       'sources': [
         'third_party/nspr/prcpucfg.h',