gpu: Fix uninitialized memory in SwapChainPresenter

|swap_chain_format_| should be initialized to prevent crashes when
calling strlen() on the return value DxgiFormatToString().  Initializing
to BGRA matches the behavior prior to adding |swap_chain_format_|.  Also
guard against such crashes by returning a valid string instead of
nullptr from DxgiFormatToString().

Bug: 1129031
Change-Id: I113bca2b7a5899dc270f97089f94c6222b2a71cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414654Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807637}

ui/gl/swap_chain_presenter.cc

@@ -133,7 +133,7 @@ const char* DxgiFormatToString(DXGI_FORMAT format) {
       return "NV12";
     default:
       NOTREACHED();
-      return nullptr;
+      return "UNKNOWN";
   }
 }
 

ui/gl/swap_chain_presenter.h

@@ -173,7 +173,7 @@ class SwapChainPresenter : public base::PowerObserver {
   gfx::Size swap_chain_size_;
 
   // Current swap chain format.
-  DXGI_FORMAT swap_chain_format_;
+  DXGI_FORMAT swap_chain_format_ = DXGI_FORMAT_B8G8R8A8_UNORM;
 
   // Whether the swap chain was reallocated, and next present will be the first.
   bool first_present_ = false;