[GTK] Fix DCHECK when themes don't provide opaque frame colors

Issue can be triggered by running Chrome with testing/xvfb.py.  The check looks
like the following:

[130960:130960:0921/173814.814345:FATAL:color_utils.cc(356)] Check failed: (((base) >> 24) & 0xFF) == SK_AlphaOPAQUE (0 vs. �)
0 0x7f05478f815d base::debug::StackTrace::StackTrace()
1 0x7f05475ff9ac base::debug::StackTrace::StackTrace()
2 0x7f054766f47a logging::LogMessage::~LogMessage()
3 0x7f0545ec4e19 color_utils::GetBlendValueWithMinimumContrast()
4 0x7f0545ec4d25 color_utils::GetColorWithMinimumContrast()
5 0x7f05319ec298 libgtkui::GtkUi::LoadGtkValues()
6 0x7f05319eb2ae libgtkui::GtkUi::Initialize()
7 0x55847d44a79b ChromeBrowserMainExtraPartsViewsLinux::ToolkitInitialized()
8 0x558478c10b15 ChromeBrowserMainParts::ToolkitInitialized()
9 0x558478c0a9c5 ChromeBrowserMainPartsLinux::ToolkitInitialized()
10 0x7f0540a4b64d content::BrowserMainLoop::InitializeToolkit()
11 0x7f0540a4e7b5 content::BrowserMainRunnerImpl::Initialize()
12 0x7f0540a39a6b content::BrowserMain()
13 0x7f05428fefe4 content::RunBrowserProcessMain()
14 0x7f05429018cb content::ContentMainRunnerImpl::Run()
15 0x7f05428f67cc content::ContentServiceManagerMainDelegate::RunEmbedderProcess()
16 0x7f0547ba37fa service_manager::Main()
17 0x7f05428fca23 content::ContentMain()
18 0x558476d46246 ChromeMain
19 0x558476d46152 main
20 0x7f051c7152b1 __libc_start_main
21 0x558476d4602a _start

R=pkasting

Change-Id: Ib6d421dc3cd0f8fb9a41afd4b1487548404f58f0
Reviewed-on: https://chromium-review.googlesource.com/1238487Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593726}

chrome/browser/ui/libgtkui/gtk_ui.h

@@ -128,9 +128,12 @@ class GtkUi : public views::LinuxUI {
                      void*,
                      GParamSpec*);
 
+  // Loads all GTK-provided settings.
+  void LoadGtkValues();
+
   // Extracts colors and tints from the GTK theme, both for the
   // ThemeService interface and the colors we send to webkit.
-  void LoadGtkValues();
+  void UpdateColors();
 
   // Sets the Xcursor theme and size with the GTK theme and size.
   void UpdateCursorTheme();