[ChromeOS]More network settings UI touch ups.

[ChromeOS]More network settings UI touch ups.

- Remove network section h3 specifics since options_page.css is using
  the latest UI now. And this makes network UI looks the same way as
  other parts. That is, buttons and network list are aligned instead of
  immediately following the section headers.
- Move "-webkit-margin-start: 15px" into the div container
  "network-general-div" and get rid of related css rules from its
  children;
- Remove stale css rules of "details-button", which no longer exists;

BUG=none
TEST=Verify ChroemOS network settings works as before. The only visual
change is the button strip and network lists are now left-aligned
instead of immediately following the section headers.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=86150

[achuithb, 2011-05-20 21:27:05]

LGTM