remove main_render_frame(), use GetMainRenderFrame() instead

remove main_render_frame(), use GetMainRenderFrame() instead

CL 93333002 added a main_render_frame() method to RenderFrameImpl,
then about a week later CL 107893003 added a virtual
GetMainRenderFrame() method with the same implementation.
To avoid confusion, we could just use the latter and remove
main_render_frame().

BUG=304341
Committed: https://crrev.com/c3335323f09a566cb6a78f424dd7c52966e13b31
Cr-Commit-Position: refs/heads/master@{#294562}

[Mostyn Bramley-Moore, 2014-09-09 17:47:23]

mostynb@opera.com changed reviewers:
+ jam@chromium.org

[Mostyn Bramley-Moore, 2014-09-09 17:49:08]

@jam: does this simplification look OK to you?

[jam, 2014-09-12 04:06:31]

lgtm, thank you

[Mostyn Bramley-Moore, 2014-09-12 07:39:11]

The CQ bit was checked by mostynb@opera.com

[commit-bot: I haz the power, 2014-09-12 07:39:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/554743009/20001

[commit-bot: I haz the power, 2014-09-12 08:10:46]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-12 08:10:47]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Mostyn Bramley-Moore, 2014-09-12 09:03:07]

The CQ bit was checked by mostynb@opera.com

[commit-bot: I haz the power, 2014-09-12 09:04:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/554743009/20001

[commit-bot: I haz the power, 2014-09-12 09:40:40]

Committed patchset #2 (id:20001) as 073ff5d0e8daf752e1bb2d5dbf68fbb132ddee80

[commit-bot: I haz the power, 2014-09-12 09:43:27]

Patchset 2 (id:??) landed as
https://crrev.com/c3335323f09a566cb6a78f424dd7c52966e13b31
Cr-Commit-Position: refs/heads/master@{#294562}