GPU context creation code duplication cleanup

Use ContextCreationAttribHelper through the context creation in both
in-process and out-of-process cases, instead of duplicate structures such
as GLInProcessContextAttribs. Enums used for serializing the attributes
are now defined in only one place, instead of three.

Remove small pieces of dead code related to share groups.
share_resources_ attrib is removed since it is not in use anywhere.

gpu_unittests are changed to better model how bind_generates_resource
gets set, and new checks are added to make sure it is set consistently
everywhere.

TEST=WebGL conformance tests (with and without --in-process-gpu),
cc_unittests, gpu_unittests

BUG=

Review URL: https://codereview.chromium.org/455783002

Cr-Commit-Position: refs/heads/master@{#289269}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289269 0039d316-1c4b-4281-b951-d872f2087c98

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index f71a2d8..7c1254a 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -618,6 +618,7 @@
     "//gpu/command_buffer/client:gles2_c_lib",
     "//gpu/command_buffer/client:gles2_implementation",
     "//gpu/command_buffer/client:gl_in_process_context",
+    "//gpu/command_buffer/common:gles2_utils",
     "//gpu/skia_bindings",
     "//skia",
     "//testing/gmock",
@@ -763,6 +764,7 @@
     "//cc/surfaces",
     "//gpu",
     "//gpu:test_support",
+    "//gpu/command_buffer/common:gles2_utils",
     "//media",
     "//testing/gmock",
     "//testing/gtest",
@@ -794,6 +796,7 @@
     "//base",
     "//gpu",
     "//gpu:test_support",
+    "//gpu/command_buffer/common:gles2_utils",
     "//media",
     "//skia",
     "//testing/gmock",