Vulkan: Add featue flag to force highp to mediump in fragment shader.

include/platform/FeaturesVk.h

@@ -496,6 +496,13 @@ struct FeaturesVk : FeatureSetBase
     // http://anglebug.com/3529
     Feature enableBufferAge = {"enableBufferAge", FeatureCategory::VulkanWorkarounds,
                                "Expose EGL_EXT_buffer_age", &members, "http://anglebug.com/3529"};
+
+    // Whether we should force any highp precision in the fragment shader to mediump.
+    // ANGLE is non-conformant if this feature is enabled.
+    Feature forceFragmentShaderPrecisionHighpToMediump = {
+        "forceFragmentShaderPrecisionHighpToMediump", FeatureCategory::VulkanWorkarounds,
+        "Forces highp precision in fragment shader to mediump.", &members,
+        "https://issuetracker.google.com/184850002"};
 };
 
 inline FeaturesVk::FeaturesVk()  = default;

src/libANGLE/renderer/vulkan/RendererVk.cpp

@@ -2366,6 +2366,10 @@ void RendererVk::initFeatures(DisplayVk *displayVk,
     // http://anglebug.com/3529
     ANGLE_FEATURE_CONDITION(&mFeatures, enableBufferAge, !isSwiftShader);
 
+    // Disabled by default. Only enable it for experimental purpose, as this will cause various
+    // tests to fail.
+    ANGLE_FEATURE_CONDITION(&mFeatures, forceFragmentShaderPrecisionHighpToMediump, false);
+
     angle::PlatformMethods *platform = ANGLEPlatformCurrent();
     platform->overrideFeaturesVk(platform, &mFeatures);
 

src/libANGLE/renderer/vulkan/ShaderVk.cpp

@@ -63,6 +63,11 @@ std::shared_ptr<WaitableCompileEvent> ShaderVk::compile(const gl::Context *conte
         compileOptions |= SH_IGNORE_PRECISION_QUALIFIERS;
     }
 
+    if (contextVk->getFeatures().forceFragmentShaderPrecisionHighpToMediump.enabled)
+    {
+        compileOptions |= SH_FORCE_SHADER_PRECISION_HIGHP_TO_MEDIUMP;
+    }
+
     // Let compiler detect and emit early fragment test execution mode. We will remove it if
     // context state does not allow it
     compileOptions |= SH_EARLY_FRAGMENT_TESTS_OPTIMIZATION;