Fix a GCC warning found by Skia.

This only seems to affect GCC's enum-compare warning,
because we were comparing an untyped enum with a size_t enum.

BUG=None 

Change-Id: I74b8315cff61344d1b7700b1c56a40d2ffce41e2
Reviewed-on: https://chromium-review.googlesource.com/698296
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Framebuffer.cpp b/src/libANGLE/Framebuffer.cpp
index 3bac235..9b595b5 100644
--- a/src/libANGLE/Framebuffer.cpp
+++ b/src/libANGLE/Framebuffer.cpp
@@ -197,13 +197,13 @@
 }
 
 // Needed to index into the attachment arrays/bitsets.
-static_assert(IMPLEMENTATION_MAX_FRAMEBUFFER_ATTACHMENTS ==
+static_assert(static_cast<size_t>(IMPLEMENTATION_MAX_FRAMEBUFFER_ATTACHMENTS) ==
                   gl::Framebuffer::DIRTY_BIT_COLOR_ATTACHMENT_MAX,
               "Framebuffer Dirty bit mismatch");
-static_assert(IMPLEMENTATION_MAX_FRAMEBUFFER_ATTACHMENTS ==
+static_assert(static_cast<size_t>(IMPLEMENTATION_MAX_FRAMEBUFFER_ATTACHMENTS) ==
                   gl::Framebuffer::DIRTY_BIT_DEPTH_ATTACHMENT,
               "Framebuffer Dirty bit mismatch");
-static_assert(IMPLEMENTATION_MAX_FRAMEBUFFER_ATTACHMENTS + 1 ==
+static_assert(static_cast<size_t>(IMPLEMENTATION_MAX_FRAMEBUFFER_ATTACHMENTS + 1) ==
                   gl::Framebuffer::DIRTY_BIT_STENCIL_ATTACHMENT,
               "Framebuffer Dirty bit mismatch");