Clean up the default gamma settings.

Clean up the default gamma settings.

This makes it a compile error to specify both SK_GAMMA_EXPONENT
and SK_GAMMA_SRGB. It also ensures that SK_GAMMA_EXPONENT will
be set consistently.

R=djsollen@google.com, reed@google.com

Committed: https://code.google.com/p/skia/source/detail?r=14143

[bungeman-skia, 2014-04-10 16:03:03]

This change will make the issue found with Android setting both of the gamma
settings a compile time error so this sort of thing doesn't happen again. Note
that we'll need to ensure that we aren't setting both Skia side before this goes
in to prevent our Android bots from breaking on this change.

[djsollen, 2014-04-10 16:06:27]

lgtm. You will want to wait and land this patch after I land my CL that does not
turn on SK_GAMMA_SRGB for Android

[reed1, 2014-04-10 19:26:27]

lgtm

[bungeman-skia, 2014-04-10 22:26:48]

Committed patchset #1 manually as r14143 (presubmit successful).