Remove some global UI defines

Remove the USE_CLIPBOARD_AURAX11 C++ define which were unused. The use_clipboard_aurax11 build variable was only used in one place (ui/base/BUILD.gn) and seems not to be useful. This is replaced by a "use_x11 && is_desktop_linux" condition.

Remove the enable_clipboard_aurax11 build arg which affected the use_ variant above and is no longer needed.

Move USE_DEFAULT_RENDER_THEME build variable and C++ define to Blink's public features.

Review-Url: https://codereview.chromium.org/2498733002
Cr-Original-Commit-Position: refs/heads/master@{#432010}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8a274fa60fec11c1c1ddbeb2ee64c3ffb60ca84c

tools/gn/docs/cookbook.md

diff --git a/tools/gn/docs/cookbook.md b/tools/gn/docs/cookbook.md
index 9a1b259..390fbf1 100644
--- a/tools/gn/docs/cookbook.md
+++ b/tools/gn/docs/cookbook.md
@@ -275,7 +275,6 @@
 | `use_athena` (0/1)                      | `use_athena` (true/false)                      | `//build/config/ui.gni`       |
 | `use_aura` (0/1)                        | `use_aura` (true/false)                        | `//build/config/ui.gni`       |
 | `use_cairo` (0/1)                       | `use_cairo` (true/false)                       | `//build/config/ui.gni`       |
-| `use_clipboard_aurax11` (0/1)           | `use_aura && use_x11`                          |                               |
 | `use_cups` (0/1)                        | `use_cups` (true/false)                        | `//build/config/features.gni` |
 | `use_dbus` (0/1)                        | `use_dbus` (true/false)                        | `//build/config/features.gni` |
 | `use_gconf` (0/1)                       | `use_gconf` (true/false)                       | `//build/config/features.gni` |