Put a bluetooth enum in an enum class to shadowing errors

A bluetooth enum was putting some common names like "SUCCESS",
"UNKNOWN" and "FAILED" in the content namespace. A recent
clang (I think) added new warnings when these got
got shadowed by other uses of the names. See full error below.

This enum was always intended to be in an enum class judging from
how it's used, so just use enum class to avoid further clashes.

This primarily affected jumbo builds but could happen to any build
which happened to include two conflicting headers in the same file.

../../content/browser/histogram_synchronizer.h:54:5: error: declaration shadows a variable in namespace 'content' [-Werror,-Wshadow]
    UNKNOWN,
    ^
../../content/browser/bluetooth/bluetooth_metrics.h:228:3: note: previous declaration is here
  UNKNOWN = 5,
  ^
In file included from gen/content/browser/browser_jumbo_1.cc:170:
In file included from ./../../content/browser/devtools/devtools_instrumentation.cc:16:
../../content/browser/frame_host/navigation_request.h:68:5: error: declaration shadows a variable in namespace 'content' [-Werror,-Wshadow]
    FAILED,
    ^
../../content/browser/bluetooth/bluetooth_metrics.h:229:3: note: previous declaration is here
  FAILED = 6,
  ^
In file included from gen/content/browser/browser_jumbo_1.cc:247:
In file included from ./../../content/browser/download/drag_download_file.cc:5:
../../content/browser/download/drag_download_file.h:53:37: error: declaration shadows a variable in namespace 'content' [-Werror,-Wshadow]
  enum State {INITIALIZED, STARTED, SUCCESS, FAILURE};
                                    ^
../../content/browser/bluetooth/bluetooth_metrics.h:223:3: note: previous declaration is here
  SUCCESS = 0,
  ^

TBR=jyasskin@chromium.org

Change-Id: I2f35c1c11262e4f071cf1d64348c9bc30c76351b
Reviewed-on: https://chromium-review.googlesource.com/c/1326507Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#606520}

content/browser/bluetooth/bluetooth_metrics.h

@@ -219,7 +219,7 @@ void RecordGetDescriptorsDescriptor(
 
 // These are the possible outcomes when performing GATT operations i.e.
 // characteristic.readValue/writeValue descriptor.readValue/writeValue.
-enum UMAGATTOperationOutcome {
+enum class UMAGATTOperationOutcome {
   SUCCESS = 0,
   NO_DEVICE = 1,
   NO_SERVICE = 2,