Add BiDi-related test cases to LocalCaretRectTest

This patch adds test cases of LocalCaretRectOfPosition() for
positions at BiDi boundaries but not logical line boundaries.

Test cases are named in format:

In{BlockDir}Block{BaseRunDir}BaseRun{Side}{NestedRun}{Deep}

- BlockDir: Direction of containing block, either 'Ltr' or 'Rtl'
- BaseRunDir: Direction of the lowest BiDi level in the block,
  either 'Ltr' or 'Rtl'
- Side: Location of caret, either 'Before' or 'After' a nested
  BiDi run
- NestedRun: Information of nested BiDi run(s) at the caret location.
- Deep: Whether the caret position in DOM is affiliated to the inner
  most text node (i.e., 'Deep') or not

See https://goo.gl/fV2sSN for more details about test generation.

Bug: 822575
Change-Id: I171c17ace0ca38ef051db7a5c6ab14cc870be009
Reviewed-on: https://chromium-review.googlesource.com/965882Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543966}

third_party/WebKit/Source/core/editing/BUILD.gn

@@ -329,6 +329,7 @@ jumbo_source_set("unit_tests") {
     "GranularityStrategyTest.cpp",
     "InlineBoxPositionTest.cpp",
     "LayoutSelectionTest.cpp",
+    "LocalCaretRectBiDiTest.cpp",
     "LocalCaretRectTest.cpp",
     "PositionIteratorTest.cpp",
     "PositionTest.cpp",