Rename buildflag_headers for consistency

This updates the remaining targets in ui/ and url/.

Targets go from foo_features to foo_buildflags
And similarly the header now has buildflags in the name.

BUG=814485
TEST=covered by build

Change-Id: I746dee03434f9bf593ea19f4868f67fa2482f14b
Reviewed-on: https://chromium-review.googlesource.com/c/1405493
Commit-Queue: Scott Violet <sky@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621679}

• chrome/browser/BUILD.gn[diff]
• chrome/browser/about_flags.cc[diff]
• chrome/browser/ui/BUILD.gn[diff]
• chrome/browser/ui/views/BUILD.gn[diff]
• chrome/browser/ui/views/nav_button_provider.h[diff]
• components/cronet/android/BUILD.gn[diff]
• components/cronet/android/cronet_library_loader.cc[diff]
• content/browser/BUILD.gn[diff]
• content/browser/gpu/gpu_data_manager_impl_private.cc[diff]
• gpu/config/BUILD.gn[diff]
• gpu/config/gpu_util.cc[diff]
• gpu/ipc/service/BUILD.gn[diff]
• gpu/ipc/service/gpu_init.cc[diff]
• ios/net/BUILD.gn[diff]
• ios/net/protocol_handler_util.mm[diff]
• net/BUILD.gn[diff]
• net/base/registry_controlled_domains/registry_controlled_domain_unittest.cc[diff]
• net/test/run_all_unittests.cc[diff]
• ui/gl/BUILD.gn[diff]
• ui/gl/gl_image_io_surface.mm[diff]
• ui/gl/gl_implementation.cc[diff]
• ui/gl/init/BUILD.gn[diff]
• ui/gl/init/gl_factory_mac.cc[diff]
• ui/gl/init/gl_initializer_android.cc[diff]
• ui/gl/init/gl_initializer_mac.cc[diff]
• ui/gl/init/gl_initializer_win.cc[diff]
• ui/gl/init/gl_initializer_x11.cc[diff]
• ui/ozone/common/BUILD.gn[diff]
• ui/ozone/common/egl_util.cc[diff]
• ui/views/BUILD.gn[diff]
• ui/views/linux_ui/linux_ui.h[diff]
• url/BUILD.gn[diff]