Re-land: Added virtualenv for depot_tools

bootstrap/virtualenv/virtualenv_embedded/activate.fish

+# This file must be used with "source bin/activate.fish" *from fish* (http://fishshell.com)
+# you cannot run it directly
+
+function deactivate  -d "Exit virtualenv and return to normal shell environment"
+    # reset old environment variables
+    if test -n "$_OLD_VIRTUAL_PATH" 
+        set -gx PATH $_OLD_VIRTUAL_PATH
+        set -e _OLD_VIRTUAL_PATH
+    end
+    if test -n "$_OLD_VIRTUAL_PYTHONHOME"
+        set -gx PYTHONHOME $_OLD_VIRTUAL_PYTHONHOME
+        set -e _OLD_VIRTUAL_PYTHONHOME
+    end
+    
+    if test -n "$_OLD_FISH_PROMPT_OVERRIDE"
+        # set an empty local fish_function_path, so fish_prompt doesn't automatically reload
+        set -l fish_function_path
+        # erase the virtualenv's fish_prompt function, and restore the original
+        functions -e fish_prompt
+        functions -c _old_fish_prompt fish_prompt
+        functions -e _old_fish_prompt
+        set -e _OLD_FISH_PROMPT_OVERRIDE
+    end
+    
+    set -e VIRTUAL_ENV
+    if test "$argv[1]" != "nondestructive"
+        # Self destruct!
+        functions -e deactivate
+    end
+end
+
+# unset irrelevant variables
+deactivate nondestructive
+
+set -gx VIRTUAL_ENV "__VIRTUAL_ENV__"
+
+set -gx _OLD_VIRTUAL_PATH $PATH
+set -gx PATH "$VIRTUAL_ENV/__BIN_NAME__" $PATH
+
+# unset PYTHONHOME if set
+if set -q PYTHONHOME
+    set -gx _OLD_VIRTUAL_PYTHONHOME $PYTHONHOME
+    set -e PYTHONHOME
+end
+
+if test -z "$VIRTUAL_ENV_DISABLE_PROMPT"
+    # fish uses a function instead of an env var to generate the prompt.
+    
+    # copy the current fish_prompt function as the function _old_fish_prompt
+    functions -c fish_prompt _old_fish_prompt
+    
+    # with the original prompt function copied, we can override with our own.
+    function fish_prompt
+        # Prompt override?
+        if test -n "__VIRTUAL_PROMPT__"
+            printf "%s%s" "__VIRTUAL_PROMPT__" (set_color normal)
+            _old_fish_prompt
+            return
+        end
+        # ...Otherwise, prepend env
+        set -l _checkbase (basename "$VIRTUAL_ENV")
+        if test $_checkbase = "__"
+            # special case for Aspen magic directories
+            # see http://www.zetadev.com/software/aspen/
+            printf "%s[%s]%s " (set_color -b blue white) (basename (dirname "$VIRTUAL_ENV")) (set_color normal) 
+            _old_fish_prompt
+        else
+            printf "%s(%s)%s" (set_color -b blue white) (basename "$VIRTUAL_ENV") (set_color normal)
+            _old_fish_prompt
+        end
+    end 
+    
+    set -gx _OLD_FISH_PROMPT_OVERRIDE "$VIRTUAL_ENV"
+end