Rename 'derived_sources' => 'generated', and assorted build cleanup

'derived_sources' is confusing and verbose, so we're renaming it to 'generated'
(AFAICT 'derived' is from legalese 'derived work', and 'sources' is from GYP.)

I also noted many small issues on the way, which I've fixed:
* Simpler build: make generating aggregate bindings files independent of
other steps (it now just depends on the list of IDL files, so we don't need
to rebuild these for other changes).
* Correct .gitignore for VS generated XML file, and remove the generated file
* Delete duplicate variable definition
* Update a stale comment
* Rename associated Python script
* Change comment to docstring in Python script
* Minor readability improvements to Python script

TBR=Nico, abarth

Review URL: https://codereview.chromium.org/144773003

git-svn-id: svn://svn.chromium.org/blink/trunk@165524 bbb929c8-8fbe-4397-9dbb-9b2b20218538