Remove duplicated header file from BUILD.gn

Duplicated header caused Visual Studio error when opening platform.vcxproj
project.

error : Cannot load project with duplicated project items:
../../../../../../../third_party/blink/renderer/platform/graphics/compositor_mutator.h
is included as 'None' and as 'None' item types.

Change-Id: Iee35d7ef24dce3083c291858546fbe5b80428ee9
Reviewed-on: https://chromium-review.googlesource.com/1107921Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Tomasz Moniuszko <tmoniuszko@opera.com>
Cr-Commit-Position: refs/heads/master@{#569181}

third_party/blink/renderer/platform/BUILD.gn

@@ -849,7 +849,6 @@ jumbo_component("platform") {
     "graphics/compositor_filter_operations.cc",
     "graphics/compositor_filter_operations.h",
     "graphics/compositor_mutator.h",
-    "graphics/compositor_mutator.h",
     "graphics/compositor_mutator_client.cc",
     "graphics/compositor_mutator_client.h",
     "graphics/compositor_mutator_impl.cc",