clang/win: Fix most -Wwriteable-strings warnings.

cloud_print/virtual_driver/win/port_monitor/port_monitor_unittest.cc

Index: cloud_print/virtual_driver/win/port_monitor/port_monitor_unittest.cc
diff --git a/cloud_print/virtual_driver/win/port_monitor/port_monitor_unittest.cc b/cloud_print/virtual_driver/win/port_monitor/port_monitor_unittest.cc
index 335503a560ef6094369395f17e36549f216904e2..a4b2e0c9534d047f8e8d29a5228cae6b2cf44495 100644
--- a/cloud_print/virtual_driver/win/port_monitor/port_monitor_unittest.cc
+++ b/cloud_print/virtual_driver/win/port_monitor/port_monitor_unittest.cc
@@ -236,7 +236,8 @@ TEST_F(PortMonitorTest, FlowTest) {
   EXPECT_TRUE(monitor2->pfnEndDocPort != NULL);
 
   // These functions should fail if we have not impersonated the user.
-  EXPECT_FALSE(monitor2->pfnStartDocPort(port_handle, L"", 0, 0, NULL));
+  EXPECT_FALSE(monitor2->pfnStartDocPort(
+      port_handle, const_cast<wchar_t*>(L""), 0, 0, NULL));
   EXPECT_FALSE(monitor2->pfnWritePort(port_handle,
                                      buffer,
                                      kBufferSize,
@@ -251,7 +252,8 @@ TEST_F(PortMonitorTest, FlowTest) {
 
   // Now impersonate so we can test the success case.
   ASSERT_TRUE(ImpersonateSelf(SecurityImpersonation));
-  EXPECT_TRUE(monitor2->pfnStartDocPort(port_handle, L"", 0, 0, NULL));
+  EXPECT_TRUE(monitor2->pfnStartDocPort(
+      port_handle, const_cast<wchar_t*>(L""), 0, 0, NULL));
   EXPECT_TRUE(monitor2->pfnWritePort(port_handle,
                                      buffer,
                                      kBufferSize,