Fixed error about base::span type conversion.

This CL fixes the build error with the following message:

         ../../net/base/mime_util.cc:522:47: error: could not convert ‘(const char* const*)(& net::{anonymous}::kStandardImageTypes)’ from ‘const char* const*’ to ‘base::span<const char* const>’
               {nullptr, base::span<const char* const>()}};
         ../../net/base/mime_util.cc:522:47: error: could not convert ‘(const char* const*)(& net::{anonymous}::kStandardAudioTypes)’ from ‘const char* const*’ to ‘base::span<const char* const>’
         ../../net/base/mime_util.cc:522:47: error: could not convert ‘(const char* const*)(& net::{anonymous}::kStandardVideoTypes)’ from ‘const char* const*’ to ‘base::span<const char* const>’

R= mmenke@chromium.org

Bug: 844285
Change-Id: I1b6483a99258d936922ffc31f08317ccb97ff8dc
Reviewed-on: https://chromium-review.googlesource.com/1075576Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: 汪 清 <wangqing-hf@loongson.cn>
Cr-Commit-Position: refs/heads/master@{#562716}

net/base/mime_util.cc

@@ -516,10 +516,10 @@ struct StandardType {
   base::span<const char* const> standard_types;
 };
 static const StandardType kStandardTypes[] = {
-    {"image/", kStandardImageTypes},
-    {"audio/", kStandardAudioTypes},
-    {"video/", kStandardVideoTypes},
-    {nullptr, base::span<const char* const>()}};
+    {"image/", base::span<const char* const>{kStandardImageTypes}},
+    {"audio/", base::span<const char* const>{kStandardAudioTypes}},
+    {"video/", base::span<const char* const>{kStandardVideoTypes}},
+    {nullptr,  base::span<const char* const>{}}};
 
 // GetExtensionsFromHardCodedMappings() adds file extensions (without a leading
 // dot) to the set |extensions|, for all MIME types matching |mime_type|.