Linux: Move base/mime_util* to base/nix. It's not used on any other platform.

tools/valgrind/memcheck/suppressions.txt

 # There are four kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 4360 matching lines @@
    fun:_ZN11PluginPrefs23CreatePluginFileSummaryERKN6webkit13WebPluginInfoE
    fun:_ZN11PluginPrefs19OnUpdatePreferencesESt6vectorIN6webkit13WebPluginInfoESaIS2_EES0_INS1_5npapi11PluginGroupESaIS6_EE
    fun:_Z16DispatchToMethodI11PluginPrefsMS0_FvSt6vectorIN6webkit13WebPluginInfoESaIS3_EES1_INS2_5npapi11PluginGroupESaIS7_EEES5_S9_EvPT_T0_RK6Tuple2IT1_T2_E
 }
 {
    bug_95902
    Memcheck:Leak
    ...
    fun:xdg_mime_init
    fun:xdg_mime_get_mime_type_from_file_name
-   fun:_ZN9mime_util15GetFileMimeTypeERK8FilePath
+   fun:*GetFileMimeTypeERK8FilePath
    fun:_ZNK3net16PlatformMimeUtil32GetPlatformMimeTypeFromExtensionERKSsPSs
    fun:_ZNK3net8MimeUtil30GetMimeTypeFromExtensionHelperERKSsbPSs
    fun:_ZNK3net8MimeUtil24GetMimeTypeFromExtensionERKSsPSs
    fun:_ZNK3net8MimeUtil19GetMimeTypeFromFileERK8FilePathPSs
    fun:_ZN3net19GetMimeTypeFromFileERK8FilePathPSs
    fun:_ZNK25SharedResourcesDataSource11GetMimeTypeERKSs
 }
 {
    bug_95931a
    Memcheck:Leak
@@ skipping 876 matching lines @@
    obj:/lib/libselinux.so.1
    obj:/lib/libselinux.so.1
 }
 {
    bug_todo_grep
    Memcheck:Leak
    fun:malloc
    obj:/bin/grep
    obj:/bin/grep
 }