Change the target type of platform_wtf to source_set.

A CL (https://codereview.chromium.org/2741343017) failed to compile on
Windows Debug try bots because files in wtf/ lacked __declspec(dllimport)
even though they need to import symbols defined in platform_wtf.

Conceptually, platform_wtf and wtf are a single link unit, thus the target
type of "component" was inappropriate; it should have been "source_set"
instead.

This change is supposed to fix the compile error mentioned above.

BUG=691465

Review-Url: https://codereview.chromium.org/2760573002
Cr-Commit-Position: refs/heads/master@{#457743}

third_party/WebKit/Source/platform/wtf/BUILD.gn

@@ -57,7 +57,7 @@ config("wtf_config") {
   }
 }
 
-component("platform_wtf") {
+source_set("platform_wtf") {
   sources = [
     "AddressSanitizer.h",
     "Alignment.h",