Fix all GCC 10 warnings

Change-Id: Ibabab265e734a4a421a38ab586e7e11905fa5da1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/64740
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/dawn_native/opengl/RenderPipelineGL.cpp b/src/dawn_native/opengl/RenderPipelineGL.cpp
index 45d9986..d5d8409 100644
--- a/src/dawn_native/opengl/RenderPipelineGL.cpp
+++ b/src/dawn_native/opengl/RenderPipelineGL.cpp
@@ -36,6 +36,7 @@
                 case wgpu::PrimitiveTopology::TriangleStrip:
                     return GL_TRIANGLE_STRIP;
             }
+            UNREACHABLE();
         }
 
         void ApplyFrontFaceAndCulling(const OpenGLFunctions& gl,
@@ -85,6 +86,7 @@
                 case wgpu::BlendFactor::OneMinusConstant:
                     return alpha ? GL_ONE_MINUS_CONSTANT_ALPHA : GL_ONE_MINUS_CONSTANT_COLOR;
             }
+            UNREACHABLE();
         }
 
         GLenum GLBlendMode(wgpu::BlendOperation operation) {
@@ -100,6 +102,7 @@
                 case wgpu::BlendOperation::Max:
                     return GL_MAX;
             }
+            UNREACHABLE();
         }
 
         void ApplyColorState(const OpenGLFunctions& gl,
@@ -166,6 +169,7 @@
                 case wgpu::StencilOperation::DecrementWrap:
                     return GL_DECR_WRAP;
             }
+            UNREACHABLE();
         }
 
         void ApplyDepthStencilState(const OpenGLFunctions& gl,