Cleanup the IPC param traits structure

Cleanup the IPC param traits structure:
-traits that are only used by chrome should be in chrome
-traits that aren't used by chrome shouldn't be in content/public
-use macros for serialization where possible
-traits that are only used by one message file should just be listed in it
-get rid of webkit_param_traits since it's not needed anymore. It was added as a hack to keep npchrome_frame.dll's size small by giving a hint to the MSVS linker, but now that there's more split between the message files because of the content split it's not necessary anymore. I've verified that npchrome_frame.dll's size (Release, non-component) size doesn't change.
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=158570

[jam, 2012-09-25 03:38:39]

(this is inspired by the issues with the current structure of the files that
were pointed out in https://codereview.chromium.org/10966050/ by jamesr and me)

[piman, 2012-09-25 03:46:07]

lgtm